External Libraries: Skip instanceof check when null in Text_Diff::_check().

On the first `foreach` loop in Text_Diff::_check()`, `$prevtype` is `null`. As `instanceof` requires the class name term to be an object or string, a fatal error is thrown:

>Fatal error: Uncaught Error: Class name must be a valid object or a string on line 279

This change:
* Adds a simple test for the `Text_Diff::_check()` method, which is how the bug was discovered as the test could never pass with the code as-is.

* Adds a defensive guard to protect against the fatal. It checks if `$prevtype` is not `null` as a pre-condition to for checking the instance. This bugfix also resolves the failing test.

Follow-up to [49194], [7747].

Props jrf, hellofromTonya.
See #62083.
Built from https://develop.svn.wordpress.org/trunk@59070


git-svn-id: http://core.svn.wordpress.org/trunk@58466 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/Text/Diff.php

@@ -276,7 +276,7 @@ class Text_Diff {
 
         $prevtype = null;
         foreach ($this->_edits as $edit) {
-            if ($edit instanceof $prevtype) {
+            if ($prevtype !== null && $edit instanceof $prevtype) {
                 trigger_error("Edit sequence is non-optimal", E_USER_ERROR);
             }
             $prevtype = get_class($edit);

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.7-alpha-59069';
+$wp_version = '6.7-alpha-59070';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.