Added check for files before generating SVG

If no files are included in stream, `gulp-svgstore` will not generate an SVG. Addresses https://github.com/w0rm/gulp-svgstore/issues/10.

Is there a changelog that should also be updated?
I think that we should return cb() here, according to [through2 docs](https://github.com/rvagg/through2#flushfunction).