Fix: do not silently ignore assign NULL to *string

AssignTo can only assign NULL to a **string. Previous code tried to
assign nil to a *string, which did nothing. Correct behavior is to
detect this as an error.

pgtype/json_test.go

@@ -129,7 +129,7 @@ func TestJSONAssignTo(t *testing.T) {
 			t.Errorf("%d: %v", i, err)
 		}
 
-		if *tt.dst == tt.expected {
+		if *tt.dst != tt.expected {
 			t.Errorf("%d: expected %v to assign %v, but result was %v", i, tt.src, tt.expected, *tt.dst)
 		}
 	}