Add ~/ as default url_prefix

[MaximeLaurenty]

I have just experienced (the hard way) that url_prefix is set to ~/ by default in the CLI but empty in the github action.
This has generated additional friction for us to set up our source-maps properly: we initially tried with the CLI and everything worked like a charm, then realized the action behaved differently.
Having both consistent (and using ~/ rather than none, because I guess it is the most common value) would probably make the setup smoother for most people.

[maximemalou]

Run into the same issue recently, Github Action should indeed have the same defaults as the CLI !