fix PyNULL typo

[ericphanson]

closes#543, closes#541

I added a test that would catch the issue:

julia>let arr = PyObjectArray([pylist([1]), pylist([2])]) PythonCall.JlWrap.pyobjectarray_finalizer(arr) end ERROR: UndefVarError:`PYNULL` not defined Stacktrace: [1] enqueue_all(ptrs::Vector{Ptr{PythonCall.C.PyObject}}) @ PythonCall.GC ~/PythonCall.jl/src/GC/GC.jl:106 [2] pyobjectarray_finalizer(x::PyObjectVector) @ PythonCall.JlWrap ~/PythonCall.jl/src/JlWrap/objectarray.jl:30 [3] top-level scope @ REPL[17]:2

[cjdoris]

Thanks! I simplified the test a bit and checked it still fails on main. I'll make a release.