TJT

Results 2 issues of TJT

see #630 But I'm not sure the value of `Struct.NulLValue` should be `null` or `undefined`, current is `undefined`, it's looks a little weird.

Assume we have enum in proto: ```protobuf enum Product { PRODUCT_UNSPECIFIED = 0; PRODUCT_A = 1; PRODUCT_B = 2; } message Example { Product product = 1; // allow undefined...