`get_comment()`, `wp_get_post_revision()`, and `get_term()` all used to return by reference. Because of this, `$null` was set to `null` so the return value would be a variable where applicable. This has not been necessary since [21792], so the `$null`s have been removed.

Props toszcze.
Fixes #24768.



git-svn-id: https://develop.svn.wordpress.org/trunk@27057 602fd350-edb4-49c9-b593-d223f7449a82