length operator on nil/boolean returns 0 instead of raising #404

Found while building an error-fidelity test battery for the perf work (#403).

return #nil and return #true both evaluate to 0. PUC-Lua 5.3 raises:

lua: attempt to get length of a nil value
lua: attempt to get length of a boolean value

Present on main independent of the peephole pass (reproduces with peephole: false and on commits before #403). Likely a missing type check in the length-operator implementation shared by both engines.