Use null instead of undefined as the prop when a custom placeholder i…

…s not set

Co-authored-by: Douglas Henri <[email protected]>

projects/plugins/jetpack/extensions/blocks/ai-assistant/extensions/with-ai-extension.tsx

@@ -500,7 +500,7 @@ const blockEditWithAiComponents = createHigherOrderComponent( BlockEdit => {
 
 				{ showAiControl && (
 					<AiAssistantInput
-						customPlaceholder={ customPlaceholder ? customPlaceholder : undefined }
+						customPlaceholder={ customPlaceholder ? customPlaceholder : null }
 						className={ className }
 						requestingState={ requestingState }
 						requestingError={ error }