Prevent non empty unions and clarify null docs #5715

Job	Run time
C++ API (coverage)	1m 12s
Rust/C++ FFI tests (asan)	2m 11s
Rust tests (miri)	1m 36s
Rust tests (tsan)	6m 2s
Rust/C++ FFI tests (tsan)	1m 57s
Rust tests (asan)	6m 37s
Mirror DuckDB to R2 / Resolve DuckDB version and check R2	6s
DuckDB libraries available in R2	3s
Mirror DuckDB to R2 / Mirror DuckDB ${{ matrix.archive }} to R2	0s
Rust tests (coverage) (tests)	6m 1s
	25m 45s

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