Fix MG katz_centrality: Check if DataFrame Arg is Not None (#4555)

[This](rapidsai/cudf#16311) PR recently introduced some API changes that broke `test_katz_centrality_mg.py` because it was using an improper method of trying to evaluate a `cudf.DataFrame` to a truth value.


This PR adds a fix to check if the `nstart` argument is set to `None`.

Authors:
  - Ralph Liu (https://github.com/nv-rliu)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)

URL: #4555

python/cugraph/cugraph/dask/centrality/katz_centrality.py

@@ -162,7 +162,7 @@ def katz_centrality(
     do_expensive_check = False
 
     initial_hubs_guess_values = None
-    if nstart:
+    if nstart is not None:
         if input_graph.renumbered:
             if len(input_graph.renumber_map.implementation.col_names) > 1:
                 cols = nstart.columns[:-1].to_list()