Small fixes for CI, sync with C

[thijsmie]

This has become a little bit of a hodgepodge of changes, but at least they are still grouped by commit!

• Commit 015deb8 syncs the TRY_CONSTRUCT flags with C PR Fixes for XTypes interoperability cyclonedds#1242
• Commit 2191b9e fixes a leak in the python sertype where a ddsi_sertype does not get unref'ed when a topic creation fails.
• Commit 672f459 fixes a small mistake in TypeObjects of unions with an Enum as discriminator
• Commit 43e9b2c fixes a mistake that only occurs on windows where the C api can return NULL from an entity name while claiming it is present. This is dealing with the symptom though, I am not exactly sure why it occurs (it doesn't reproduce locally for me, only on CI...)

This should make the CI fully green again, provided the fuzzer doesn't find anything random 😄

[thijsmie]

@eboasson the CI on master had a failure in the fuzzer due to a case in a union being called get which name-clashed with a IdlUnion method, an interesting case indeed 😅.