Block Bindings: Fix empty strings placeholders in post meta bindings (W…

…ordPress#65089)

* Contemplate empty strings as truthy

* Add e2e test for empty values

Co-authored-by: zaguiini <zaguiini@git.wordpress.org>
Co-authored-by: SantosGuillamot <santosguillamot@git.wordpress.org>

packages/e2e-tests/plugins/block-bindings.php

@@ -41,6 +41,16 @@ function gutenberg_test_block_bindings_registration() {
 			'default'      => '#url-custom-field',
 		)
 	);
+	register_meta(
+		'post',
+		'empty_field',
+		array(
+			'show_in_rest' => true,
+			'type'         => 'string',
+			'single'       => true,
+			'default'      => '',
+		)
+	);
 	register_meta(
 		'post',
 		'_protected_field',

packages/editor/src/bindings/post-meta.js

@@ -22,7 +22,7 @@ export default {
 		for ( const [ attributeName, source ] of Object.entries( bindings ) ) {
 			// Use the key if the value is not set.
 			newValues[ attributeName ] =
-				meta?.[ source.args.key ] || source.args.key;
+				meta?.[ source.args.key ] ?? source.args.key;
 		}
 		return newValues;
 	},

test/e2e/specs/editor/various/block-bindings.spec.js

@@ -1278,6 +1278,38 @@ test.describe( 'Block bindings', () => {
 				).toHaveText( 'fallback value' );
 			} );
 
+			test( 'should show the prompt placeholder in field with empty value', async ( {
+				editor,
+			} ) => {
+				await editor.insertBlock( {
+					name: 'core/paragraph',
+					attributes: {
+						content: 'paragraph default content',
+						metadata: {
+							bindings: {
+								content: {
+									source: 'core/post-meta',
+									args: { key: 'empty_field' },
+								},
+							},
+						},
+					},
+				} );
+
+				const paragraphBlock = editor.canvas.getByRole( 'document', {
+					// Aria-label is changed for empty paragraphs.
+					name: 'Add empty_field',
+				} );
+
+				await expect( paragraphBlock ).toBeEmpty();
+
+				const placeholder = paragraphBlock.locator( 'span' );
+				await expect( placeholder ).toHaveAttribute(
+					'data-rich-text-placeholder',
+					'Add empty_field'
+				);
+			} );
+
 			test( 'should not show the value of a protected meta field', async ( {
 				editor,
 			} ) => {