allow bundling/minifying in otherwise empty assets

This pull request addresses an issue when trying to bundle/minify a file that only contain Sprocket-directives.

In current master, file_put_contents(...) report a size of 0 after minifying a file with only comments (as they are removed). As 0 is a falsy value, the check !0 is true and Sprocket throws an exception.

This commit solves the issue by specifically checking if the returned value is false.