Fix possible bug if reusing tagged struct
Fix possible bug if reusing tagged struct

Unlikely to be encountered, but: if caller is re-using a tagged struct
and unmarshalling a nil value into it, make sure to actually set it to
{“”, nil}.

NeilFraser/CodeCity,google/CodeCity,google/CodeCity,NeilFraser/CodeCity,NeilFraser/CodeCity,google/CodeCity,NeilFraser/CodeCity,google/CodeCity