GH-427 Fix issue in getTransBlockNum where if except was None it woul…

…d fail with no_block instead of returning the block number.

tests/Node.py

@@ -154,7 +154,7 @@ def getTransBlockNum(trans):
             cntxt.add("processed")
             cntxt.add("action_traces")
             cntxt.index(0)
-            if cntxt.hasKey("except"):
+            if not cntxt.isSectionNull("except"):
                 return "no_block"
             return cntxt.add("block_num")