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.

git-svn-id: https://develop.svn.wordpress.org/trunk@59070 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

src/wp-includes/Text/Diff.php

@@ -276,7 +276,7 @@ function _check($from_lines, $to_lines)
 
         $prevtype = null;
         foreach ($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);

tests/phpunit/tests/diff/Text_Diff_Check_Test.php

@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * Tests for WP native customizations added to the Text_Diff::check() method.
+ *
+ * @group diff
+ *
+ * @covers Text_Diff::_check
+ */
+final class Text_Diff_Check_Test extends WP_UnitTestCase {
+
+	const FILE_A = array(
+		'Line 1',
+		'Line 2',
+		'Line 3',
+	);
+
+	const FILE_B = array(
+		'Line 11',
+		'Line 2',
+		'Line 13',
+	);
+
+	public static function set_up_before_class() {
+		require_once ABSPATH . 'wp-includes/Text/Diff.php';
+	}
+
+	/**
+	 * Disable WP specific set up as it is not needed.
+	 */
+	public function set_up() {}
+
+	public function test_check_passes_when_passed_same_input() {
+		$diff = new Text_Diff( 'auto', array( self::FILE_A, self::FILE_B ) );
+		$this->assertTrue( $diff->_check( self::FILE_A, self::FILE_B ) );
+	}
+}