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.
Built from https://develop.svn.wordpress.org/trunk@51800


git-svn-id: http://core.svn.wordpress.org/trunk@51407 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51799';
+$wp_version = '5.9-alpha-51800';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.