@import (inline, optional) fails with source maps enabled

[mmelvin0]

If I do the following:

@import (inline, optional) 'file.css';

When file.css is missing, LESS still tries to generate a source map and fails in SourceMapOutput.add since this._contentsMap[fileInfo.filename] is null for the optional, missing import.

Disabling source maps (or supplying file.css) allows the build to complete successfully.

[mmelvin0]

I'm willing to try my hand at fixing this... any hints as to how I can tell during source map generation that an import was in fact optional?

[lukeapage]

A simple guard around this._contentsMap[fileInfo.filename] might be okay.. That or better, look at where sourcemap gets its list of fileInfo's and make optional not present imports not add to that list (prob. In import manager or the import node)

[mmelvin0]

I originally changed what Import.eval() was returning so it didn't have a null filename, but opted for this change to ImportVisitor instead as it seemed simpler and uses already existing hooks for skipping an import.