Disallow empty source name in standard json

[axic]

No description provided.

[axic]

The reason for this change was that outputSelection would be confused, because empty source name refers to SourceUnit-level outputs (aka AST).

However, jsonCompiler needs to be amended, because the old compile() entry point has only a source code and uses an empty file name. The same applies to compileJSON() it allows empty file names. (Need to add test for this in solc-js just to be sure too.)

[chriseth]

What is the state of this PR?

[axic]

jsonCompiler needs to be reviewed how it fits in for the legacy use cases as there it might require right now the use of an empty file name.

I also have another change set which cleans this up in CompilerStack.

[axic]

The reason for this change was that outputSelection would be confused, because empty source name refers to SourceUnit-level outputs (aka AST).

Revisited this again and empty source name is not an issue, empty contract name is.