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
 }
 

server/public/model/post_test.go

@@ -977,3 +977,16 @@ func TestPostForPlugin(t *testing.T) {
 		require.NotNil(t, pluginPost.GetProp("requested_features"))
 	})
 }
+
+func TestPostPriority(t *testing.T) {
+	p := &Post{
+		Metadata: &PostMetadata{},
+	}
+	require.False(t, p.IsUrgent())
+
+	p.Metadata.Priority = &PostPriority{}
+	require.False(t, p.IsUrgent())
+
+	p.Metadata.Priority.Priority = NewPointer(PostPriorityUrgent)
+	require.True(t, p.IsUrgent())
+}