mod_bookmarks2: Fix handling of nick and password elements This form of child retrieval fails when the stanza elements internally don't have an 'xmlns' attribute, which can happen sometimes for some reason, including when they have been constructed via the stanza builder API. When that is the case then the explicit namespace arguemnt does not match the nil value of the internal attribute. Calling `:get_child()` without the namespace argument does the right thing here, with both nil and the parent namespace as valid values for the internal attribute.

This module implements the [Server
Optimization](https://xmpp.org/extensions/xep-0410.html#serveroptimization)
part of [XEP-0410: MUC Self-Ping]

# Usage

The module is loaded on MUC components:

```lua
Component "muc.example.com" "muc"
modules_enabled = {
    "muc_ping";
}
```

# Configuration

No options.

# Compatibility

It should work with Prosody up until 0.10.x.

Prosody 0.11.x and trunk natively supports XEP-0410 so this module is **not** needed.