Add a new bucket drop reason for no data.

Bug: 157155330
Test: m -j
Change-Id: I17bb6b9f9692bb6e185c2dc0414dcb166e6e120c
diff --git a/cmds/statsd/src/stats_log.proto b/cmds/statsd/src/stats_log.proto
index 1121392..6bfa267 100644
--- a/cmds/statsd/src/stats_log.proto
+++ b/cmds/statsd/src/stats_log.proto
@@ -212,6 +212,8 @@
       MULTIPLE_BUCKETS_SKIPPED = 7;
       // Not an invalid bucket case, but the bucket is dropped.
       BUCKET_TOO_SMALL = 8;
+      // Not an invalid bucket case, but the bucket is skipped.
+      NO_DATA = 9;
   };
 
   message DropEvent {