FIX(plot): failing if steps not a list/ is none...

backported from #29

Author: ankostis

graphkit/plot.py

@@ -33,7 +33,7 @@ def get_node_name(a):
         kw = {}
         if isinstance(nx_node, str):
             # Only DeleteInstructions data in steps.
-            if nx_node in steps:
+            if steps and nx_node in steps:
                 kw = {"color": "red", "penwidth": 2}
 
             # SHAPE change if in inputs/outputs.