Bug 904467 - error when %recipients is empty (e.g. after bugmail_recipients empties it)

Bugzilla/BugMail.pm

@@ -186,6 +186,8 @@ sub Send {
                             { bug => $bug, recipients => \%recipients,
                               users => \%user_cache, diffs => \@diffs });
 
+    # We should not assume %recipients to have any entries.
+    if (scalar keys %recipients) {
         # Find all those user-watching anyone on the current list, who is not
         # on it already themselves.
         my $involved = join(",", keys %recipients);
@@ -202,6 +204,7 @@ sub Send {
             }
             push(@{$watching{$watch->[0]}}, $watch->[1]);
         }
+    }
 
     # Global watcher
     my @watchers = split(/[,\s]+/, Bugzilla->params->{'globalwatchers'});