multi valued property model lets JackRabbit determine fallback type, instead of defaulting to STRING

With relaxed property definitions, the property type needs to be specified if a zero-length array is set. This does not happen in the multi-valued property model, leaving JackRabbit the choice to find a suitable type. With the hippostd:relaxed mixin, this is usually the type DOUBLE.

It would be better if the model forces the STRING type when it cannot determine an explicit type.