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-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.