handle case if madlc predictions are all empty

[LucZot]

Catches the problem of stacking multi-pose predictions that are all empty; in this case it returns empty poses as it would do for the single animal case. (Solves error also in here: https://forum.image.sc/t/dlc-live-multianimal-what-is-affinity-in-inferenceutil-py-empty-assemblies-bug-or-bad-model/69972)