[Security Solution] [Security Assistant] Fixes an unable to load page error in the Security assistant (#194488)

### [Security Solution] [Security Assistant] Fixes an unable to load page error in the Security assistant

This PR fixes an `Unable to load page error` in the Security Assistant, introduced in <https://github.com/elastic/kibana/pull/193645>, where a check that appeared to be optional was removed.

- The issue occurs when the assistant is asked a question that is likely to require anonymization, for example: `which alerts should I look at first?`
- After the error occurs, re-opening the assistant for the same conversation may re-trigger the error
- As a workaround, clear Kibana local storage for the current user, or via the browser's dev tools, specifically delete the `elasticAssistantDefault.lastConversationId` setting illustrated by the screenshot below:

![local_storage_key](https://github.com/user-attachments/assets/882f68e2-a253-49d7-84ad-fe2326bbfe20)

### Steps to reproduce

To reproduce:

1) Ask the assistant a question that is likely to cause anonymized alerts to be returned, for example: `which alerts should I look at first?`

**Expected result**

- Anonymized alerts are displayed

**Actual result**

- An `Unable to load page` error is displayed, as illustrated by the screenshot below:

![unable_to_load_page](https://github.com/user-attachments/assets/54006a07-5a79-45d6-97cb-f36e0a71361f)

- The error details include text similar to the output below:

```
TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at replaceAnonymizedValuesWithOriginalValues (http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:60965:14)
    at transformMessageWithReplacements (http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:100511:158)
    at transformMessage (http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:100612:41)
    at http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:100638:32
    at Array.map (<anonymous>)
    at getComments (http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:100595:45)
    at http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:68997:15
    at updateMemo (http://localhost:5601/XXXXXXXXXXXX/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:369829:19)
    at Object.useMemo (http://localhost:5601/XXXXXXXXXXXX/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:370375:16)


The above error occurred in AssistantComponent:
    at AssistantComponent (http://localhost:5601/XXXXXXXXXXXX/bundles/plugin/securitySolution/1.0.0/securitySolution.chunk.lazy_application_dependencies.js:68776:3)
    at div
    at http://localhost:5601/XXXXXXXXXXXX/bundles/kbn-ui-shared-deps-npm/kbn-ui-shared-deps-npm.dll.js:160008:73
    at div
    at http://loc
```

- The following video illustrates the error:

<https://github.com/user-attachments/assets/e96d7ac0-032d-49d6-88be-a8add6547fb1>

x-pack/packages/kbn-elastic-assistant-common/impl/data_anonymization/helpers/index.test.ts

@@ -123,6 +123,30 @@ describe('helpers', () => {
       expect(result).toEqual(messageContent);
     });
 
+    it('returns the original messageContent if replacements is null', () => {
+      const messageContent =
+        'User {{ user.name 3541b730-1dce-4937-b63f-0d618ea1cc5f }} added a member to the Administrators group on host {{ host.name b222e892-431e-4e4f-9295-2ba92ef9d12d }}';
+
+      const result = replaceAnonymizedValuesWithOriginalValues({
+        messageContent,
+        replacements: null, // <-- null
+      });
+
+      expect(result).toEqual(messageContent);
+    });
+
+    it('returns the original messageContent if replacements is undefined', () => {
+      const messageContent =
+        'User {{ user.name 3541b730-1dce-4937-b63f-0d618ea1cc5f }} added a member to the Administrators group on host {{ host.name b222e892-431e-4e4f-9295-2ba92ef9d12d }}';
+
+      const result = replaceAnonymizedValuesWithOriginalValues({
+        messageContent,
+        replacements: undefined, // <-- undefined
+      });
+
+      expect(result).toEqual(messageContent);
+    });
+
     it('replaces multiple occurrences of the same replacement key', () => {
       const messageContent =
         'User {{ user.name 3541b730-1dce-4937-b63f-0d618ea1cc5f }} added a member to the Administrators group on host {{ host.name b222e892-431e-4e4f-9295-2ba92ef9d12d }}, which is unusual because {{ user.name 3541b730-1dce-4937-b63f-0d618ea1cc5f }} is not a member of the Administrators group.';

x-pack/packages/kbn-elastic-assistant-common/impl/data_anonymization/helpers/index.ts

@@ -40,13 +40,15 @@ export const replaceAnonymizedValuesWithOriginalValues = ({
   replacements,
 }: {
   messageContent: string;
-  replacements: Replacements;
+  replacements: Replacements | null | undefined;
 }): string =>
-  Object.keys(replacements).reduce((acc, key) => {
-    const value = replacements[key];
+  replacements != null
+    ? Object.keys(replacements).reduce((acc, key) => {
+        const value = replacements[key];
 
-    return acc.replaceAll(key, value);
-  }, messageContent);
+        return acc.replaceAll(key, value);
+      }, messageContent)
+    : messageContent;
 
 export const replaceOriginalValuesWithUuidValues = ({
   messageContent,