Fixes#2651
First, our OSC parser didn't allow blank OSC 0 or 2 requests. This
should be allowed and this fixes that with a test.
Second, it seems many terminals (iTerm2, Kitty) treat setting a blank
title as resetting to whatever the default title is rather than
explicitly setting it as blank. If a program wants a title to be blank
they should send a single space. This commit follows this behavior.