Tests: Fix "null to non-nullable" deprecation notice in Tests_Admin_IncludesPlugin::test_get_plugin_files_folder().

The `Tests_Admin_IncludesPlugin::_create_plugin()` expects the first parameter to be a text string to be written to a plugin file using `fwrite()`.

Passing null causes a `fwrite(): Passing null to parameter #2 ($data) of type string is deprecated` notice.

Ref: https://www.php.net/manual/en/function.fwrite

Follow-up to [31002]. [41806].

Props jrf, hellofromTonya.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51800 602fd350-edb4-49c9-b593-d223f7449a82

Author: hellofromtonya

tests/phpunit/tests/admin/includesPlugin.php

@@ -366,7 +366,7 @@ public function test_get_plugin_files_single() {
 	public function test_get_plugin_files_folder() {
 		$plugin_dir = WP_PLUGIN_DIR . '/list_files_test_plugin';
 		@mkdir( $plugin_dir );
-		$plugin = $this->_create_plugin( null, 'list_files_test_plugin.php', $plugin_dir );
+		$plugin = $this->_create_plugin( '', 'list_files_test_plugin.php', $plugin_dir );
 
 		$sub_dir = trailingslashit( dirname( $plugin[1] ) ) . 'subdir';
 		mkdir( $sub_dir );