Marshal/Unmarshal missing UDT fields as null instead of failing in unsafe mode

We can't return an error in case a field is added to the UDT,
otherwise existing code would break by simply altering the UDT in the
database. For extra fields at the end of the UDT put nulls to be in
line with gocql, but also python-driver and java-driver.

In gocql it was fixed in d2ed1bb74f

udt.go

@@ -39,26 +39,24 @@ func makeUDT(value reflect.Value, mapper *reflectx.Mapper, unsafe bool) udt {
 
 func (u udt) MarshalUDT(name string, info gocql.TypeInfo) ([]byte, error) {
 	value, ok := u.field[name]
-
-	var data []byte
-	var err error
 	if ok {
-		data, err = gocql.Marshal(info, value.Interface())
-		if err != nil {
-			return nil, err
-		}
+		return gocql.Marshal(info, value.Interface())
 	}
-
-	return data, err
+	if u.unsafe {
+		return nil, nil
+	}
+	return nil, fmt.Errorf("missing name %q in %s", name, u.value.Type())
 }
 
 func (u udt) UnmarshalUDT(name string, info gocql.TypeInfo, data []byte) error {
 	value, ok := u.field[name]
-	if !ok && !u.unsafe {
-		return fmt.Errorf("missing name %q in %s", name, u.value.Type())
+	if ok {
+		return gocql.Unmarshal(info, data, value.Addr().Interface())
 	}
-
-	return gocql.Unmarshal(info, data, value.Addr().Interface())
+	if u.unsafe {
+		return nil
+	}
+	return fmt.Errorf("missing name %q in %s", name, u.value.Type())
 }
 
 // udtWrapValue adds UDT wrapper if needed.