C++: do not set any highlighting ranges on empty lines.

This can happen when the semantic highlighter (which runs in a future)
calculates a range for a document that has been changed in the meantime.

Task-number: QTCREATORBUG-7995
Change-Id: Ibcd1a6ae840e08570a6b6ea4caf411e799ec26c8
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>