ART fixes

[taniabogatsch]

This PR includes the following changes:

• tests to cover UNION child types in the ART (Index on union breaks database with Invalid Type [UNION(...)]: Invalid type for index key after it was created #7725)
• more fixes to avoid serializing empty ART buffers
• disabling test/sql/index/art/memory/test_art_varchar.test_slow on 32bit

@Mytherin, we probably want to merge this after merging your fix for the unaligned accesses into the validity_t. As the tests that I added here trigger it.

[Mytherin]

Thanks for the PR! Can you include the patch for the validity alignment in this PR?

[Mytherin]

Thanks!