codefmt shouldn't call FormatRanges with empty a:ranges

As noted in #48, we seem to have some cases where a formatter's `FormatRanges()` will be invoked with an empty `a:ranges`. Either that, or we're unnecessarily checking for emptiness in the clang-format formatter.

Unless I'm missing something, this doesn't seem like it's useful; the interface with formatters should say that this is impossible, and we should remove the checks we have.