UnifiedHighlighter: distinguish no field from no passages in field

The UnifiedHighlighter does not distinguish between highlighting a field that is absent on a document, and highlighting a field that is present but no passages were found (can happen for a variety of reasons) – both cases produce null. While not a huge deal in general, it's an annoyance. It can be useful for the user/client to detect the content was present but not highlightable so that it might take some default action, like returning the whole field value (possibly to process in some way) or highlighting some other field.