GODRIVER-3434 Avoid initializing null data given custom decoder (#1903)

Author: prestonvasquez

bson/default_value_decoders.go

@@ -1166,6 +1166,12 @@ func valueUnmarshalerDecodeValue(_ DecodeContext, vr ValueReader, val reflect.Va
 		return ValueDecoderError{Name: "ValueUnmarshalerDecodeValue", Types: []reflect.Type{tValueUnmarshaler}, Received: val}
 	}
 
+	if vr.Type() == TypeNull {
+		val.Set(reflect.Zero(val.Type()))
+
+		return vr.ReadNull()
+	}
+
 	if val.Kind() == reflect.Ptr && val.IsNil() {
 		if !val.CanSet() {
 			return ValueDecoderError{Name: "ValueUnmarshalerDecodeValue", Types: []reflect.Type{tValueUnmarshaler}, Received: val}

bson/unmarshaling_cases_test.go

@@ -197,6 +197,25 @@ type unmarshalerNonPtrStruct struct {
 
 type myInt64 int64
 
+var _ ValueUnmarshaler = (*myInt64)(nil)
+
+func (mi *myInt64) UnmarshalBSONValue(t byte, b []byte) error {
+	if len(b) == 0 {
+		return nil
+	}
+
+	if Type(t) == TypeInt64 {
+		i, err := newValueReader(TypeInt64, bytes.NewReader(b)).ReadInt64()
+		if err != nil {
+			return err
+		}
+
+		*mi = myInt64(i)
+	}
+
+	return nil
+}
+
 func (mi *myInt64) UnmarshalBSON(b []byte) error {
 	if len(b) == 0 {
 		return nil