Fix return type for modResource::getTVValue() (#16567)

Correct the union of return types in documentation, so it's understood
by IDEs.

### What does it do?
IDEs like PhpStorm saw this method as returning only `null`. This change
gets them to see `null` or `mixed` as the return types.

### Why is it needed?
Described above.

### How to test
Use this method in an IDE; confirm it doesn't have any red lines under
the usages.

### Related issue(s)/PR(s)
N/A

core/src/Revolution/modResource.php

@@ -896,7 +896,7 @@ public function hasChildren()
      *
      * @param mixed $pk Either the ID of the TV, or the name of the TV.
      *
-     * @return null/mixed The value of the TV for the Resource, or null if the
+     * @return null|mixed The value of the TV for the Resource, or null if the
      * TV is not found.
      */
     public function getTVValue($pk)