General: Fix problematic string to array parsing.

WordPress has historically often used code like `preg_split( '/[\s,]+/', $var )` to parse a string of comma-separated values into an array. However, this approach was causing an empty string to not be parsed into an empty array as expected, but rather into an array with the empty string as its sole element.

This was among other areas causing problems in the REST API where passing an empty request parameter could cause that request to fail because, instead of it being ignored, that parameter would be compared against the valid values for it, which typically do not include an empty string.

Props david.binda, sstoqnov.
Fixes #43977.

Built from https://develop.svn.wordpress.org/trunk@44546


git-svn-id: http://core.svn.wordpress.org/trunk@44377 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44545';
+$wp_version = '5.1-alpha-44546';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.