PostController: editing a discussion or poll would result in a hidden CategoryID input with an empty value whenever 'Display root categories as headings' was checked. This did not cause issues when editing a regular discussion, which was fortunate, but when editing a poll the form would error out due to a null CategoryID.
Merge branch 'stage' of github.com:vanilla/vanilla into feature/postcontroller-fix
Merge pull request #2000 from vanilla/feature/postcontroller-fix

PostController: editing a discussion or poll would result in a missing CategoryID error