Unmarshaling empty messages is incompatible with proto.Unmarshal

When unmarshaling an empty message embedded in another message, vtprotobuf is allocating the message, but proto.Unmarshal is using a typed nil.

Steps to reproduce:

syntax = "proto3";
package repro;

message TopLevel {
  message Empty {}
  Empty embedded = 1;
}

func TestEmbeddedEmpty(t *testing.T) {
	m := &pb.TopLevel{Embedded: &pb.TopLevel_Empty{}}

	// Marshal it with protobuf.
	protobuf, err := proto.Marshal(m)
	if err != nil {
		t.Fatal(err)
	}

	// Unmarshal it with protobuf.
	pbm := &pb.TopLevel{}
	if err := proto.Unmarshal(protobuf, m); err != nil {
		t.Fatal(err)
	}

	// Unmarshal it with vtprotobuf.
	vtm := &pb.TopLevel{}
	if err := vtm.UnmarshalVT(protobuf); err != nil {
		t.Fatal(err)
	}

	fmt.Printf("%#v\n\n", pbm)
	fmt.Printf("%#v\n", vtm)

	require.True(t, pbm.EqualVT(vtm), "EqualVT")
	require.True(t, proto.Equal(pbm, vtm), "proto.Equal")
}

Output:

&proto.TopLevel{state:impl.MessageState{NoUnkeyedLiterals:pragma.NoUnkeyedLiterals{}, DoNotCompare:pragma.DoNotCompare{}, DoNotCopy:pragma.DoNotCopy{}, atomicMessageInfo:(*impl.MessageInfo)(nil)}, sizeCache:0, unknownFields:[]uint8(nil), Embedded:(*proto.TopLevel_Empty)(nil)}

&proto.TopLevel{state:impl.MessageState{NoUnkeyedLiterals:pragma.NoUnkeyedLiterals{}, DoNotCompare:pragma.DoNotCompare{}, DoNotCopy:pragma.DoNotCopy{}, atomicMessageInfo:(*impl.MessageInfo)(nil)}, sizeCache:0, unknownFields:[]uint8(nil), Embedded:(*proto.TopLevel_Empty)(0xc00011fbc0)}

--- FAIL: TestEmbeddedEmpty (0.00s)
    main_test.go:37: 
        	Error Trace:	main_test.go:37
        	Error:      	Should be true
        	Test:       	TestEmbeddedEmpty
        	Messages:   	EqualVT
FAIL
exit status 1

Start with the TestEmbeddedEmpty reproduction and compare proto.Unmarshal with the generated UnmarshalVT entry point for the embedded TopLevel_Empty message. Trace how each path handles an empty embedded message; done means both paths produce equivalent results and the EqualVT and proto.Equal checks pass.