Default `$parent` in `category_exists()` should default to null rather than 0.

[29863] made the corresponding change in `term_exists()`. Failure to change the
default value in `category_exists()` meant that an unspecified value for
`$parent` would limit results to top-level categories.

Includes unit tests and corrected function documentation.

Props hissy.
Fixes #30975 for trunk.
Built from https://develop.svn.wordpress.org/trunk@31140


git-svn-id: http://core.svn.wordpress.org/trunk@31121 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/taxonomy.php

@@ -11,14 +11,17 @@
 //
 
 /**
- * {@internal Missing Short Description}}
+ * Check whether a category exists.
  *
  * @since 2.0.0
  *
- * @param int|string $cat_name
- * @return int
+ * @see term_exists()
+ *
+ * @param int|string $cat_name Category name.
+ * @param int        $parent   Optional. ID of parent term.
+ * @return mixed
  */
-function category_exists($cat_name, $parent = 0) {
+function category_exists( $cat_name, $parent = null ) {
 	$id = term_exists($cat_name, 'category', $parent);
 	if ( is_array($id) )
 		$id = $id['term_id'];

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31139';
+$wp_version = '4.2-alpha-31140';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.