Improve json serializer

[ppazos]

The serializer shouldn't assume the input is valid, and should verify required attributes have a value instead of assuming they are present, so we avoid giving an exception to the user from the not found method, and instead we give our exception about the missing required data.

Exception in thread "main" groovy.lang.MissingMethodException: No signature of method: com.cabolabs.openehr.formats.OpenEhrJsonSerializer.serializeNullObject() is applicable for argument types: (null) values: [null]

Might need to fix the xml serializer too.