Do not output empty metric reports.

Test: statsd unit test passed.

Change-Id: I296edbe621eaea72ad2e1dfe656698302614c0f3
diff --git a/cmds/statsd/src/metrics/EventMetricProducer.cpp b/cmds/statsd/src/metrics/EventMetricProducer.cpp
index 821d8ea..25c86d0 100644
--- a/cmds/statsd/src/metrics/EventMetricProducer.cpp
+++ b/cmds/statsd/src/metrics/EventMetricProducer.cpp
@@ -102,6 +102,9 @@
 
 void EventMetricProducer::onDumpReportLocked(const uint64_t dumpTimeNs,
                                              ProtoOutputStream* protoOutput) {
+    if (mProto->size() <= 0) {
+        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);
     protoOutput->write(FIELD_TYPE_INT64 | FIELD_ID_END_REPORT_NANOS, (long long)dumpTimeNs);