Query Loop: Check for postTypeFromContext before using it (WordPress#…

…66655)

Co-authored-by: mikachan <mikachan@git.wordpress.org>
Co-authored-by: ellatrix <ellatrix@git.wordpress.org>

packages/block-library/src/post-template/edit.js

@@ -188,7 +188,7 @@ export default function PostTemplateEdit( {
 			// When we preview Query Loop blocks we should prefer the current
 			// block's postType, which is passed through block context.
 			const usedPostType =
-				postTypeFromContext !== 'page'
+				postTypeFromContext && postTypeFromContext !== 'page'
 					? postTypeFromContext
 					: previewPostType || postType;
 			return {