[3.2] Fix incorrect serializing of std::optional when value is not provided - (GH #1189)

[greg7mdp]

When serializing a std::optional field, and a value is not provided for the std::optional, we need to add into the serialization the flag specifying that a missing value is serialized. Prior to this PR, the flag was missing.

[greg7mdp]

@heifner @arhag One concern I have is that this PR changes the serialization output for structs containing std::optional, when a value is omitted for one or more std::optional.
My first thought is that it may not be an issue, because the deserialization of these would have failed anyways, but I thought it would be worth mentioning.

[heifner]

@heifner @arhag One concern I have is that this PR changes the serialization output for structs containing std::optional, when a value is omitted for one or more std::optional. My first thought is that it may not be an issue, because the deserialization of these would have failed anyways, but I thought it would be worth mentioning.

I don't anticipate any issue here.

[linh2931]

first should be second in the comment.

[linh2931]

should be last.