TinyMCE: prevent error in the wptextpattern plugin when the first child node in a paragraph is not a text node.

See #31441;

git-svn-id: https://develop.svn.wordpress.org/trunk@32960 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/tinymce/plugins/wptextpattern/plugin.js

@@ -88,6 +88,10 @@
 				}
 			}
 
+			if ( ! child ) {
+				return;
+			}
+
 			if ( ! child.nodeValue ) {
 				child = child.nextSibling;
 			}