Allow branchPrefix prompt to handle empty input during init

[jtheuerkauf]

If a branch prefix is not wanted for jj-spr setting up branches, it does accept "" or '':

$ jj-spr init
  [ ... clip ... ]
  ❓  What prefix should be used when naming Pull Request branches?
      Good practice is to begin with 'spr/' as a general namespace for
      spr-managed Pull Request branches. Continuing with the GitHub user name is
      a good idea, so there is no danger of names clashing with those of other users.
      The prefix should end with a good separator character (like '/' or '-'),
      since commit titles will be appended to this prefix.
Branch prefix: ''
$ jj config e --repo
[spr]
githubRemoteName = "origin"
githubRepository = "jtheuerkauf/learning-jj"
githubMasterBranch = "trunk"
branchPrefix = ''

If I clear the prefix and leave the input empty, it just restores the default. Also, is the empty string shown above actually an empty string, or will SPR try to use '' as part of a branch name?

I would like that prompt to allow blank input to achieve the same result, or to specify in the text above the prompt that ""/'' will set no-prefix.

$ jj-spr init
  [ ... ]
  ❓  What prefix should be used when naming Pull Request branches?
[ ... ]
Branch prefix:
$ jj config e --repo
[spr]
githubRemoteName = "origin"
githubRepository = "jtheuerkauf/learning-jj"
githubMasterBranch = "trunk"
branchPrefix =