codefmt shouldn't call FormatRanges with empty a:ranges

[malcolmr]

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.

[dbarnett]

I think I would want some kind of empty check in the clang-format code regardless, because otherwise there's a correctness issue: FormatRanges([X, Y]) means "format ranges X and Y", but FormatRanges([]) means "format entire buffer". Changing the empty check to a hard precondition and changing codefmt to never call with empty ranges should be fine.