[Discover] Fix horizontal rule in Additional display settings popover (#177460)

- Noticed in https://github.com/elastic/kibana/pull/176064

## Summary

This PR renders `EuiHorizontalRule` only between available options.

Before:
<img width="494" alt="Screenshot 2024-02-21 at 15 57 24"
src="f718008e-82fb-48f2-9e30-bc607e571e44">

After:
<img width="491" alt="Screenshot 2024-02-21 at 15 56 47"
src="fe6b2bf1-93b6-4913-bd40-d7187a06e7ee">

packages/kbn-unified-data-table/src/components/data_table_additional_display_settings.tsx

@@ -90,9 +90,10 @@ export const UnifiedDataTableAdditionalDisplaySettings: React.FC<
     setActiveSampleSize(sampleSize); // reset local state
   }, [sampleSize, setActiveSampleSize]);
 
-  return (
-    <>
-      {onChangeHeaderRowHeight && onChangeHeaderRowHeightLines && (
+  const settings = [];
+
+  if (onChangeHeaderRowHeight && onChangeHeaderRowHeightLines) {
+    settings.push(
       <RowHeightSettings
         rowHeight={headerRowHeight}
         rowHeightLines={headerRowHeightLines}
@@ -104,10 +105,11 @@ export const UnifiedDataTableAdditionalDisplaySettings: React.FC<
         data-test-subj="unifiedDataTableHeaderRowHeightSettings"
         maxRowHeight={5}
       />
-      )}
-      {onChangeRowHeight && onChangeRowHeightLines && (
-        <>
-          <EuiHorizontalRule margin="s" />
+    );
+  }
+
+  if (onChangeRowHeight && onChangeRowHeightLines) {
+    settings.push(
       <RowHeightSettings
         rowHeight={rowHeight}
         rowHeightLines={rowHeightLines}
@@ -118,11 +120,11 @@ export const UnifiedDataTableAdditionalDisplaySettings: React.FC<
         onChangeRowHeightLines={onChangeRowHeightLines}
         data-test-subj="unifiedDataTableRowHeightSettings"
       />
-        </>
-      )}
-      {onChangeSampleSize && (
-        <>
-          <EuiHorizontalRule margin="s" />
+    );
+  }
+
+  if (onChangeSampleSize) {
+    settings.push(
       <EuiFormRow label={sampleSizeLabel} display="columnCompressed">
         <EuiRange
           compressed
@@ -136,8 +138,17 @@ export const UnifiedDataTableAdditionalDisplaySettings: React.FC<
           data-test-subj="unifiedDataTableSampleSizeInput"
         />
       </EuiFormRow>
-        </>
-      )}
+    );
+  }
+
+  return (
+    <>
+      {settings.map((setting, index) => (
+        <React.Fragment key={`setting-${index}`}>
+          {index > 0 && <EuiHorizontalRule margin="s" />}
+          {setting}
+        </React.Fragment>
+      ))}
     </>
   );
 };