Fix fallback behavior on invalid/empty line ranges.

[msuozzo]

The existing logic behaves poorly when the output of _LineRangesToSet is empty (note: not None, an empty set). Instead of restricting formatting to the empty set of lines (i.e. suppressing all formats), the current behavior lifts all line constraints and formats the whole file. It's perhaps not a bug but I think it's surprising behavior and the improper response to unexpected input.

[bwendling]

Do you have a testcase for this?

[msuozzo]

Done

[thiagowfx]

Done

Where?

[bwendling]

This technically should never happen because _LineRangesToSet returns a non-null set if and only if there are line ranges specified.

[msuozzo]

Done

Where?

oops! it never got pushed.

Regardless, I'm fine abandoning this given it's no longer relevant for our use-case.