Navigation Link: Fix "[object Object]" in link preview for untitled entities

For a page with an empty title the entity record's title is
`{ raw: '', rendered: '' }`, so `entityRecord?.title?.rendered` is an empty
string and the link preview's title computation fell through to
`entityRecord?.title` -- the title object itself -- which renders as
"[object Object]".

Only fall back to `entityRecord.title` when it is a plain string (some entity
types expose it that way) so the object is never surfaced.

Before WordPress#77170 this rendered an object as a React child and crashed the block
("This block has encountered an error and cannot be previewed"). WordPress#77170
wrapped the preview title in stripHTML, which coerced the object to the
harmless "[object Object]" string; this addresses the underlying cause.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: taipeicoder

packages/block-library/src/navigation-link/shared/test/use-link-preview.test.js

@@ -500,6 +500,22 @@ describe( 'useLinkPreview', () => {
 			expect( result.current.title ).toBe( 'Test Category' );
 		} );
 
+		it( 'should not surface the title object for an empty title', () => {
+			const { result } = renderHook( () =>
+				useLinkPreview( {
+					url: 'https://example.com/page',
+					// Untitled page: an empty `title.rendered` object.
+					entityRecord: { title: { raw: '', rendered: '' } },
+					type: 'page',
+					hasBinding: false,
+					isEntityAvailable: true,
+				} )
+			);
+
+			// Falls back to a string, not the title object (or "[object Object]").
+			expect( result.current.title ).toBe( 'https://example.com/page' );
+		} );
+
 		it( 'should use entityRecord.name for taxonomy terms', () => {
 			const { result } = renderHook( () =>
 				useLinkPreview( {

packages/block-library/src/navigation-link/shared/use-link-preview.js

@@ -225,7 +225,9 @@ export function useLinkPreview( {
 
 	const title =
 		entityRecord?.title?.rendered ||
-		entityRecord?.title ||
+		// Only use a string `title`, so an empty `title.rendered` doesn't
+		// surface the { raw, rendered } object ("[object Object]").
+		( typeof entityRecord?.title === 'string' && entityRecord.title ) ||
 		entityRecord?.name;
 
 	// Fetch rich URL data if we don't have a title. Internal links should have passed a title.