mixing with optional fields: core dump --experimental_allow_proto3_optional #7463

message PbTest {
    optional string test_string = 1;
    optional bool test_bool = 2;
    optional int32 test_int32 = 3;
}

>>> from cylc.flow.data_messages_pb2 import PbWorkflow, PbTest
>>> test1 = PbTest(test_string='foo', test_bool=True, test_int32=1234)
>>> test1
test_string: "foo"
test_bool: true
test_int32: 1234
>>> test2 = PbTest(test_string='', test_bool=False, test_int32=0)
>>> test2
test_string: ""
test_bool: false
test_int32: 0
>>> test1.MergeFrom(test2)
>>> test1
test_string: ""
test_bool: false
test_int32: 0

message PbTest {
    string no_opt_string = 1;
    optional string test_string = 2;
    //optional bool test_bool = 2;
    //optional int32 test_int32 = 3;
}

>>> from cylc.flow.data_messages_pb2 import PbWorkflow, PbTest
>>> test1 = PbTest()
>>> test1
[libprotobuf FATAL google/protobuf/generated_message_reflection.cc:1015] CHECK failed: (has_bit_index) != (~0u): 
terminate called after throwing an instance of 'google::protobuf::FatalException'
  what():  CHECK failed: (has_bit_index) != (~0u): 
Aborted (core dumped)

message PbMeta {
    string title = 1;
    string description = 2;
    string URL = 3;
    repeated string user_defined = 4;
}
message PbTest {
    //repeated string no_opt_strings = 1;
    optional string test_string = 2;
    Meta meta = 3;
    //optional bool test_bool = 2;
    //optional int32 test_int32 = 3;
}