Do not generate empty files nor empty source map files

[dotnetwise]

Empty css generated files should not have the sourcemap comment at the end, rather they should not be generated on the disk at all.
Of course, no .map should be generated for empty files as well.

There are many candidates to such less files such as mixins, variables etc.

Currently you get a css file like this (and an associated source map too)

/*# sourceMappingURL=mixins.css.map */

[lukeapage]

yep, I agree, thanks for the bug report.