(maint) Remove failure for empty generated tags

Commit 0436eb1e0997a6f19ea734923a1b82625a4034f8 added an extra failure
condition for any generated tags that end up being empty strings. This
was to get it to handle that the same way that it handles a simple empty
string tag. However, this could break any installations that generate
"::" separated tags that could create 4 ":" in a row ("a::::b"), and it
also doesn't address enough of the odd tagging behavior to make it worth
it. This commit reverts that change and adds some more tests to cover
some more of the ways in which setting tags behaves.