Allow pushing with only remote, no branch name

**Is your feature request related to a problem? Please describe.**
I often set up my git repos with an `origin` remote and an additional `fork` remote.
Then, when I want to make a PR, I can fork off a new branch, commit, then run `git push fork`.
Git will then automatically push my current branch.
In lazygit (0.42.0), bringing up the push panel and deleting the branch name will come up with the error: `Invalid upstream. Must be in the format ' '`

**Describe the solution you'd like**
Update the matching conditions for a valid push target