[perf] No error on empty traceset.

Change-Id: I1285b5f2ad8c83e98a32ba33a89b5be344b7dfb6
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/337398
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
diff --git a/perf/go/regression/detector.go b/perf/go/regression/detector.go
index 4f55be7..dd4e834 100644
--- a/perf/go/regression/detector.go
+++ b/perf/go/regression/detector.go
@@ -199,6 +199,9 @@
 		after := len(df.TraceSet)
 		message := fmt.Sprintf("Filtered Traces: Num Before: %d Num After: %d Delta: %d", before, after, before-after)
 		sklog.Info(message)
+		if after == 0 {
+			continue
+		}
 
 		k := p.request.Alert.K
 		if k <= 0 || k > maxK {