if pattern is blank, don't try to set a value

I have something weird going on with my the [current-page:title] | [site:name] pattern for the global Page title metatag. I debugged it for a while, but couldn't find the real cause.

It made me dig into the metatag generation process, though, and I started to wonder what would happen if the pattern were blanked out. My expectation is that metatag will not add anything if the pattern is blank but in fact it adds an empty title tag.