fix custom clustering by checking explicitly for series (#318)

scripts/cluster_network.py

@@ -269,7 +269,7 @@ def clustering_for_n_clusters(n, n_clusters, custom_busmap=False, aggregate_carr
     else:
         raise AttributeError(f"potential_mode should be one of 'simple' or 'conservative' but is '{potential_mode}'")
 
-    if not custom_busmap:
+    if not isinstance(custom_busmap, pd.Series):
         busmap = busmap_for_n_clusters(n, n_clusters, solver_name, focus_weights, algorithm)
     else:
         busmap = custom_busmap