A few small things that were missing in wasm-smith's Wasm GC support

[fitzgen]

See each commit for details.

Part of bytecodealliance/wasmtime#10596

FWIW, I don't think we want to invest in non-null reference support, at least for the foreseeable future. It would require at minimum:

• A types analysis to figure out which types are uninhabitable due to cycles of non-null references (struct A has a non-null reference of a B; struct B has a non-null reference of an A)
• Fixing all the spots where we assume we can create a default value if needed, since we can't always do that in this scenario
• Tracking local initialization