REST API: Fix autovivification deprecation notice in `WP_Test_REST_Widgets_Controller::set_up()`.

If the `'widget_testwidget'` option does not exist, `false` was returned from `get_option()`. The `set_up()` logic expects an `array()` and assigns values to keys without checking for an array. The automatic creation of an array (autovivification) triggers a `Deprecated: Automatic conversion of false to array is deprecated in` deprecation notice on PHP 8.1.

This commit:
- Fixes the deprecation notice by making the default value an empty array.
- Moves getting the option within the conditional where it's needed.
- Provides a micro-optimization by only getting the options when the conditions are correct for processing.
- Makes the code consistent within the `set_up()` for both `get_option()` instances.

Follow-up to [51029].

Props jrf, hellofromTonya, BinaryKitten.
See #53635.
Built from https://develop.svn.wordpress.org/trunk@51830


git-svn-id: http://core.svn.wordpress.org/trunk@51437 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51829';
+$wp_version = '5.9-alpha-51830';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.