[Effects] call_ref traps when the target is null (#4826)
[Effects] call_ref traps when the target is null (#4826)

This is not observable in practice atm since call_ref also does a call,
which has even more effects. However, future optimizations might benefit
from this, and it is more consistent to avoid marking the instruction as
trapping if it can't.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen