pr_pause just switches back to master if no remote branch

[cderv]

Fixes #714

Small fix as you can see. I just return early if no remote.

One think I am not sure: Should there be a message or no message as there is no remote to check ?

> pr_pause()
✔ No remote branch yet for local branch 'fix-714-pause-no-remote'.
✔ Switching back to 'master' branch

or

> pr_pause()
✔ Switching back to 'master' branch

[jennybc]

Thanks!