GODRIVER-3436 Avoid initializing null data given custom decoder (#1902)

Author: prestonvasquez

bson/bsoncodec/default_value_decoders.go

@@ -1521,6 +1521,12 @@ func (dvd DefaultValueDecoders) ValueUnmarshalerDecodeValue(_ DecodeContext, vr
 		return ValueDecoderError{Name: "ValueUnmarshalerDecodeValue", Types: []reflect.Type{tValueUnmarshaler}, Received: val}
 	}
 
+	if vr.Type() == bsontype.Null {
+		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

@@ -199,6 +199,25 @@ type unmarshalerNonPtrStruct struct {
 
 type myInt64 int64
 
+var _ ValueUnmarshaler = (*myInt64)(nil)
+
+func (mi *myInt64) UnmarshalBSONValue(t bsontype.Type, bytes []byte) error {
+	if len(bytes) == 0 {
+		return nil
+	}
+
+	if t == bsontype.Int64 {
+		i, err := bsonrw.NewBSONValueReader(bsontype.Int64, bytes).ReadInt64()
+		if err != nil {
+			return err
+		}
+
+		*mi = myInt64(i)
+	}
+
+	return nil
+}
+
 func (mi *myInt64) UnmarshalBSON(bytes []byte) error {
 	if len(bytes) == 0 {
 		return nil