Tolerate null source_elapsed in trace events

For PYTHON-48

CHANGELOG.rst

@@ -18,6 +18,8 @@ Bug Fixes
 * Handle data that is already utf8-encoded for UTF8Type values
 * Fix token-aware routing for tokens that fall before the first node token in
   the ring and tokens that exactly match a node's token
+* Tolerate null source_elapsed values for Trace events.  These may not be
+  set when events complete after the main operation has already completed.
 
 Other
 -----

cassandra/query.py

@@ -489,7 +489,10 @@ class TraceEvent(object):
         self.description = description
         self.datetime = datetime.utcfromtimestamp(unix_time_from_uuid1(timeuuid))
         self.source = source
-        self.source_elapsed = timedelta(microseconds=source_elapsed)
+        if source_elapsed is not None:
+            self.source_elapsed = timedelta(microseconds=source_elapsed)
+        else:
+            self.source_elapsed = None
         self.thread_name = thread_name
 
     def __str__(self):