Do not add an empty value from config credential.helper

Below is current git message when a local config credential.helper has
an empty value. Please skip an empty value.

$ git push --force origin master
git: 'credential-' is not a git command. See 'git --help'.

Did you mean this?
        credential
Total 0 (delta 0), reused 0 (delta 0)
To https://user@github.com/user/myrepo.git
 + d23aa6a...3405990 master -> master (forced update)