Butler: fix global tags applied to empty map.

logdog/client/butler/bundler/stream.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package bundler
 
 import (
         "fmt"
         "sync"
         "sync/atomic"
@@ skipping 347 matching lines @@
 
         return s.appendErr
 }
 
 func (s *streamImpl) setAppendErrorLocked(err error) {
         s.appendErr = err
 
         s.closeLocked()
         s.signalDataConsumed()
 }
+
+func (s *streamImpl) streamDesc() *logpb.LogStreamDescriptor {
+        return s.c.template.Desc
+}