BugFix: Optional enum when it is null

[REASY]

This PR is fixing #6830

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[REASY]

@googlebot I signed it!

[REASY]

Should I bump up the minor version

flatbuffers/include/flatbuffers/base.h

Line 146 in 550d2f9

#define FLATBUFFERS_VERSION_MINOR 0

?

[CasperN]

Should I bump up the minor version?

Seems reasonable to me, we just started being strict with semver. That said, I'm not familiar with the update process, perhaps there may be more places to touch? @aardappel would know

[REASY]

@CasperN @aardappel I git blamed https://github.com/google/flatbuffers/blob/master/include/flatbuffers/base.h#L147 to see whether FLATBUFFERS_VERSION_REVISION was ever changed, didn't find anything. According to SemVer 2.0 one should increment PATCH version when you make backwards compatible bug fixes. Am I right that FLATBUFFERS_VERSION_REVISION is equivalent to the PATCH version and I should increment that one instead of FLATBUFFERS_VERSION_MINOR?

[CasperN]

Hmm, well, i think there's been a breaking change since 2.0.0 somewhere, so its not important to bump the semver. We'll just advance to 3.0.0 at the next release

[CasperN]

I'll squash and merge once CI passes. Thank you for contributing!