`git dm` misleading error if 0 branches

[Rudxain]

It prints "fatal: branch name required", if there are 0 branches to delete, because the pipe isn't "feeding" it any names.

According to GNU's xargs --help (from findutils), this is an easy fix:

-r, --no-run-if-empty        if there are no arguments, then do not run COMMAND;
                                 if this option is not given, COMMAND will be
                                 run at least once

IDK, if this works for other implementations