Fix operator== for flatbuffers::optional

[mikudehuane]

flatbuffers::Optional behaves different from std::optional when both lhs and rhs are nullopt

flatbuffers::Optional<int> lhs = flatbuffers::nullopt;
flatbuffers::Optional<int> rhs = flatbuffers::nullopt;
std::cout << (lhs == rhs) << std::endl;  // false

std::optional<int> lhs = std::nullopt;
std::optional<int> rhs = std::nullopt;
std::cout << (lhs == rhs) << std::endl;  // true

This pull request simply modifies operator==

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