BUG: Fix segfault in lib.isnullobj
Weird segfault arises when you call `lib.isnullobj` with an array that uses 0-field values to mean `None`.
Changed input to be a `Python` object (i.e. no typing), and the segfault went away.  Discovered when there were segfaults in printing a `DataFrame` containing such an array.

Closes #13717.
