GoWireStruct: correctly check for zero value when decoding
GoWireStruct: correctly check for zero value when decoding

Before we compared with None, but the zero value for an ArrayType is
not None -- it's a namedtuple:

  ArrayType(CommonType=CommonType(Name=b'', Id=0), Elem=0, Len=0)

Since pointers are flattened on the sender side, and since we have no
concept of a type on the receiver side, we must compare with the zero
value instead of simply comparing with None.

mgeisler/pygob