This is adding a trailing unconditionally, so this is no different than adding a ' ' at last component of your layout.
Ad documented in https://github.com/arl/gitmux#layout-components you can add anything to your layout, like:

layout: [branch, .., remote-branch, divergence, flags, ' ']

Also, I thought that your issue was solved with #93.
The other case as far as I'm aware of when gitmux can output an empty string is if you're not in a git directory, which you can easily get with git rev-parse --is-inside-work-tree in your tmux.conf. I'm not sure the feature you're asking is gitmux's responsibility to solve

Totally fair, I'm not sure exactly where I got confused with why I was getting more spaces than I wanted.

Seems this is irrelevant, thanks for clarifying! I'll be playing with git worktrees soon.

Loving how gitmux is shaping up btw, I'll let you know if I come up with any more ideas. 😄

Sure. Please do!!!
Cheers