Add AllowEmptyInput flag to PromptOpts

Most of our prompts don't (shouldn't) allow empty input, but most callers didn't
check, and would run into cryptic errors when the user pressed enter at an empty
prompt (e.g. when creating a new branch). Now we simply don't allow hitting
enter in this case, and show an error toast instead.

This behavior is opt-out, because there are a few cases where empty input is
supported (e.g. creating a stash).

pkg/gui/controllers/helpers/credentials_helper.go

@@ -41,6 +41,7 @@ func (self *CredentialsHelper) PromptUserForCredential(passOrUname oscommands.Cr
 
 				return nil
 			},
+			AllowEmptyInput: true,
 		})
 
 		return nil