Creating an empty tag renders TAB-completion useless.

Example:

* Title     :a::b:

Pressing TAB in org-set-tags-command will now insert a colon `:`, which is
quite puzzling to see.
I thought something was wrong w/ my bindings till I noticed this was
happening only in one of my files.

Can such tags be ignored/disallowed as they hold no evident purpose?

Regards,
Dmitrii