[Security Solution] [Attack discovery] Fix mistake in default attack discovery graph state (#213651)

## Summary

Fix mistake instroduced in [this
PR](https://github.com/elastic/kibana/pull/205553/files#diff-7a3a06cc740e97fe6941bb05a878a18561ed194946d3f28fadeb3f2f38dc1013R50).
The default state of continuePrompt was not migrated correctly.

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x-pack/solutions/security/plugins/elastic_assistant/server/lib/attack_discovery/graphs/default_attack_discovery_graph/state/index.test.ts

@@ -124,6 +124,12 @@ describe('getDefaultGraphState', () => {
     expect(graphAnnotation.spec.unrefinedResults.value).toBeNull();
   });
 
+  it('returns the expected default continuePrompt state', () => {
+    const graphAnnotation = getDefaultGraphAnnotation({ prompts });
+
+    expect(graphAnnotation.spec.continuePrompt.value).toBe(prompts.continue);
+  });
+
   it('returns the expected default end', () => {
     const graphAnnotation = getDefaultGraphAnnotation({ prompts });
 

x-pack/solutions/security/plugins/elastic_assistant/server/lib/attack_discovery/graphs/default_attack_discovery_graph/state/index.ts

@@ -47,7 +47,7 @@ export const getDefaultGraphAnnotation = ({ end, filter, prompts, start }: Optio
     }),
     continuePrompt: Annotation<string, string>({
       reducer: (x: string, y?: string) => y ?? x,
-      default: () => '',
+      default: () => prompts.continue,
     }),
     end: Annotation<string | null | undefined>({
       reducer: (x?: string | null, y?: string | null) => y ?? x,