Add a default ctor for sapi::v::Proto

If the default ctored proto message cannot be serialized that likely means it's not usable with SAPI at all (at least for proto3).\
This is so that one can easily construct a `sapi::v:Proto` to get result out of a sandboxee.

PiperOrigin-RevId: 630951119
Change-Id: Ie3d558ddaba77ab4aa045e4324b674a127a7424b

sandboxed_api/var_proto.h

@@ -28,6 +28,7 @@
 
 #include "absl/base/attributes.h"
 #include "absl/base/macros.h"
+#include "absl/log/check.h"
 #include "absl/log/log.h"
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
@@ -51,6 +52,8 @@ class Proto : public Var {
   static_assert(std::is_base_of<google::protobuf::MessageLite, T>::value,
                 "Template argument must be a proto message");
 
+  Proto() : wrapped_var_(SerializeProto(T{}).value()) {}
+
   Proto(PrivateToken, std::vector<uint8_t> data)
       : wrapped_var_(std::move(data)) {}