[bug] Fix Tmux Config Parsing

[coreyja]

Hey again! I've made a few changes in my fork, and wanted to contribute them back! The first one here is a bug fix I ran into with newer versions of tmux

I was running into issues with parsing the my tmux config after a tmux upgrade. Unfortunately I didn't record the version I was upgrading too, for documentation

What I found when digging in was that tmux was reporting a config option that did not have a value. This was breaking the config parsing in muxed. Since muxed only actually checks for a few config options, and not the value less ones I was finding. The easiest fix seemed to be to default to an empty string value in this case, which this PR implements!