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
Should we do instance checks here or have something similar to `AbstractFieldMapper.isSortable()` to decide if a mapper supports a certain feature, is `isSupportingNullValue`?