Do not create empty nick element in bookmarks

[guusdk]

When creating a bookmark, Converse (from older versions up to and including the current latest release, v10.1.7), appears to always add a nick element, even when there is no nickname.

This leads to bookmarks, like these:

<storage xmlns="storage:bookmarks">
	<conference autojoin="false" jid="myRoom@conference.example.org" name="Test Room">
		<nick/>
	</conference>
</storage>

It appears that Converse adds an empty nick element for pre-existing bookmarks (possibly defined in other clients in the past), while adding/modifying antoher bookmark in the collection of bookmarks.

This arguably breaks the specifications. Other clients fail to parse these empty elements, leading to bugs. Eg: igniterealtime/Smack#485

[jcbrand]

Thanks, should be fixed now in master.