[test] account for first timers in outlier calculation

When it's the first time a test is run, there'll not be previous results
to compare against. The script that calculates outliers was not prepared
for this situation. This commit just fixes that.

scripts/benchmark/check-for-outliers.py

@@ -145,6 +145,8 @@ def detect_bad_outlier(test, mean, extra):
 
 
 def _detect_outlier(test, name, value, list):
+    if not list:
+        return 0
     mean = _mean(list)
     std = _std(list)
     result = 0