Do not output empty metric reports.

Test: statsd unit test passed.

Change-Id: I296edbe621eaea72ad2e1dfe656698302614c0f3
diff --git a/cmds/statsd/src/metrics/GaugeMetricProducer.cpp b/cmds/statsd/src/metrics/GaugeMetricProducer.cpp
index 17305e3..24dc5b0 100644
--- a/cmds/statsd/src/metrics/GaugeMetricProducer.cpp
+++ b/cmds/statsd/src/metrics/GaugeMetricProducer.cpp
@@ -125,6 +125,9 @@
     VLOG("gauge metric %lld report now...", (long long)mMetricId);
 
     flushIfNeededLocked(dumpTimeNs);
+    if (mPastBuckets.empty()) {
+        return;
+    }
 
     protoOutput->write(FIELD_TYPE_INT64 | FIELD_ID_ID, (long long)mMetricId);
     protoOutput->write(FIELD_TYPE_INT64 | FIELD_ID_START_REPORT_NANOS, (long long)mStartTimeNs);