Server: raise ValidationError if field of parameter message is not set

Fields of Protobuf messages are optional:

• Command_Parameters(Param1=String(value="")) is a parameter message containing one field Param1 containing an empty string (serialized as two bytes "0x0A00")
• Command_Parameters() is an empty message (serialized as zero bytes)

When a parameter message is missing a field, a ValidationError should be raised.

Part B:

All Command Parameter fields are mandatory, the SiLA Server MUST throw a Validation Error if the fields are missing.