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.