sources: clarify behavior of null case

[takikawa]

An entry in sources can be null according to the spec, but the full behavior of this isn't clear. For example, if the sources entry is null, then you might guess from the spec that if a sourcesContent entry is provided then it's an "anonymous" original source.

But this doesn't really work if you have multiple such anonymous sources, because browsers collapse them into one. e.g., with the following source map:

{
  "version": 3,
  "names": [],
  "sources": [null, null],
  "sourcesContent": ["foobar1\nline2", "foobar2\nbazquux"],
  "mappings":"AAAA,SCCS"
}

In Chrome and Firefox, you'll only see one source file in the debugger named null that points to the first content.

Also, if you have the same map but delete sourcesContent:

{
  "version": 3,
  "names": [],
  "sources": [null, null],
  "mappings":"AAAA,SCCS"
}

and have a file named null sitting around, browsers may also load that as a source as well.

[jkup]

This might be a good one to bring to plenary and ask browsers what they would like / if they would be willing to change their behavior here!