Make CliGit and JGit implementations of getRemoteUrl(String, String) more similar

CliGit did not accept a blank argument (empty string or a null), while JGit did.

JGit did not return a useful value for any non-blank argument.

There are still differences between the behaviors of the two implementations, but the tests and implementation changes confirm they are becoming more similar, and they remove one case of NotImplementedInJGit.