Test-suite: fix geometric mean of empty list

  The geometric mean computation panicked when it was given
  an empty list, which happens when there are no baselines.

  Instead, we should simply return 1.

testsuite/driver/runtests.py

@@ -363,8 +363,11 @@ def cleanup_and_exit(exitcode):
         shutil.rmtree(tempdir, ignore_errors=True)
     exit(exitcode)
 
-def geometric_mean(xs: List[float]) -> float:
-    return reduce(operator.mul, xs)**(1. / len(xs))
+def geometric_mean(xs):
+    if len(xs) > 0:
+      return reduce(operator.mul, xs)**(1. / len(xs))
+    else:
+      return 1
 
 def tabulate_metrics(metrics: List[PerfMetric]) -> None:
     abbrevLen = get_abbrev_hash_length()