Fix an error raised when some node data are empty.

Sometimes node data could be empty (for example, the application
uses gevent.spawn() heavily, but not sure why). In that case, several
points of linesman raise KeyError.

It fixes node data to have 'totaltime' key before graph data are used.

linesman/middleware.py

@@ -413,6 +413,11 @@ def prepare_graph(source_graph, cutoff_time, break_cycles=False):
     # Always use a copy for destructive changes
     graph = source_graph.copy()
 
+    # Some node data could be empty dict
+    for node, data in graph.nodes(data=True):
+        if not data:
+            data['totaltime'] = 0
+
     max_totaltime = max(data['totaltime']
                         for node, data in graph.nodes(data=True))
     for node, data in graph.nodes(data=True):