MM-62085: Handle nil post priority (#29496)

https://mattermost.atlassian.net/browse/MM-62085

```release-note
NONE
```

server/public/model/post.go

@@ -935,6 +935,10 @@ func (o *Post) IsUrgent() bool {
 		return false
 	}
 
+	if postPriority.Priority == nil {
+		return false
+	}
+
 	return *postPriority.Priority == PostPriorityUrgent
 }