Fix deprioritization of regex matches in ModelRegistry

If you had an HTML element that was matched by two models, one with
a direct string match and one with a regex match, then the string
match would beat the regex match (which is correct) if they both
specified a tag name, but the regex match would win (which is wrong)
if they both didn't specify a tag name.

The fix is to only check for tagName === null if we're in tag-agnostic
mode (tag === '').

Change-Id: I9943611111e4c4ff498cdd95b7b3e72f95fb413b