fix(stats): return None when current response time not in percentile …

…window

This prevents the data from being displayed on the UI chart during the
response time calculation's indeterminate state.

Issue: locustio#1852

locust/stats.py

@@ -589,7 +589,7 @@ def get_current_response_time_percentile(self, percent):
                 percent,
             )
         # if time was not in response times cache window
-        return 0
+        return None
 
     def percentile(self):
         if not self.num_requests:

locust/test/test_stats.py

@@ -584,7 +584,7 @@ def test_get_current_response_time_percentile_outside_cache_window(self):
         s = StatsEntry(self.stats, "/", "GET", use_response_times_cache=True)
         # an empty response times cache, current time will not be in this cache
         s.response_times_cache = {}
-        self.assertEqual(0, s.get_current_response_time_percentile(0.95))
+        self.assertEqual(None, s.get_current_response_time_percentile(0.95))
 
     def test_diff_response_times_dicts(self):
         self.assertEqual(