post_exists returns 0 and null

[barrykooij]

The function post_exists, located in wp-admin/includes/post.php, returns 0 if the arguments are empty but returns null if the query has no result. Because of this the output of the function can't be strictly checked (===).

Changed the 0 return to null.

[barrykooij]

post.php patch

[markoheijnen]

See also: #21606

[nacin]

In 22410:

Always return an integer from post_exists(). props barrykooij. fixes #21607.