Return sass.NULL when not handling the file

[lencioni]

According to the node-sass documentation:

If an importer does not want to handle a particular path, it should
return sass.NULL.

https://github.com/sass/node-sass#importer--v200---experimental

This will allow other importers to properly handle the import. Some more
discussion on this can be found at:

webpack/sass-loader#170 (comment)

Since the documentation says that this is true as of 3.0.0, that's where
I set the dependency version.

[pmowrer]

Nice work!

[lencioni]

Thanks! Unfortunately, this only seems to resolve half of the problem I was running into. Do you know of a way for me to use this importer with sass-loader so that things like @import '~my_config.json' will work? It needs to use webpack to resolve what the ~ means, but then it needs to use this importer to resolve the JSON itself.

[lencioni]

Maybe I need to better utilize node-sass includePaths option instead of relying on webpack to resolve everything.

[lencioni]

@pmowrer Any chance you could cut a new release? :)

[kenjiqq]

This change seems to break some imports when node-sass in run from another package, for instance grunt-sass.
Because two packages that use node-sass each have they own node-sass in their node_modules the sass.NULL value you return here will not have the same object reference as the node-sass module that called the importer.
So the following check in node-sass fails because it't not the same NULL object and node-sass interprets that as if the importer is handling all imports.

var result = importer.call(options.context, file, prev, done);

if (result) {
    done(result === module.exports.NULL ? null : result);
}

[trotzig]

@kenborge do you think what you mention above could be the cause of #15 as well?

[pmowrer]

Sounds like we need some guidance from node-sass. Thanks for creating the following ticket @kenborge. Linking it here so we can keep tabs on the discussion:
sass/node-sass#1291