Remove use of ArrayRef(std::nullopt_t) constructor (#59973)
This is deprecated on LLVM 21 and in almost all cases this could be
replaced with a simple `{}` which is both shorter and clearer that it
was an array argument expected here. Only exception is constructor of
`jl_cgval_t` due to an ambiguity.
As far as I can tell, all the places that was replaced has always been
taking an `ArrayRef` so this should work on older LLVM versions as well.
