Completion mapping type throws a misleading error on null value

When the mapper service gets a null value for a field, it tries to parse it with the mapper of any previously seen field, if that mapper happens to be CompletionMapper, it throws an error, as it only accepts certain fields.

Closes #6399
@spinscale Thanks for the suggestion, I think `isSupportingNullValue` is a cleaner way to do this. Changed as suggested.