Update mattermost library

vendor/github.com/mattermost/logr/v2/formatters/gelf.go

@@ -79,7 +79,7 @@ type gelfRecord struct {
 func (gr gelfRecord) MarshalJSONObject(enc *gojay.Encoder) {
 	enc.AddStringKey(GelfVersionKey, GelfVersion)
 	enc.AddStringKey(GelfHostKey, gr.getHostname())
-	enc.AddStringKey(GelfShortKey, gr.Msg())
+	enc.AddStringKey(GelfShortKey, gr.safeMsg("-")) // Gelf requires a non-empty `short_message`
 
 	if gr.level.Stacktrace {
 		frames := gr.StackFrames()
@@ -131,6 +131,15 @@ func (gr gelfRecord) IsNil() bool {
 	return gr.LogRec == nil
 }
 
+// safeMsg returns the log record Message field or an alternate string when msg is empty.
+func (gr gelfRecord) safeMsg(alt string) string {
+	s := gr.Msg()
+	if s == "" {
+		s = alt
+	}
+	return s
+}
+
 func (g *Gelf) getHostname() string {
 	if g.Hostname != "" {
 		return g.Hostname