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 !

[andreashouben]

Took me two days and a thread in your Discord to figure this out. Please consider this issue.

[szokeasaurusrex]

When implementing this, we should not hardcode the default URL prefix as ~. Instead, if the URL prefix is not set in the action, we should not pass any value to the CLI. This way, we ensure that the CLI's default value is used

[andreashouben]

Creating the expected behavior with less code is always to be preferred :-)