Skip mappings with nil importer in Source::Map#to_json

[djudd]

This was already happening in the case where options[:type] != :inline, but would cause an error in the case of :inline

I'm not sure exactly under what circumstances this condition was triggered, or if it's a valid state, but it occurred in my production Rails app, and an equivalent monkeypatch got everything working.

[chriseppstein]

It seems to me that the issue is with this line. Skipping when there's no importer shouldn't be necessary in the case of :inline as we have the contents of the file (E.g. this may be the case when reading from stdin).

@nex3 knows this part of the code better than me, so I'll defer to her.

[nex3]

I think this is correct. Even though we should have the source text for the file somewhere, we don't have any means of accessing it in this code except through the importer, which may not exist. If we want source maps generated for importer-less templates, it may be necessary to refactor the code such that they always have an importer of some sort.