remove extra <ul> in walker class

[hailin]

As reported by James R Whitehead of interconnectit.com on wp-hackers:

I'm wondering why the html generated by wp_list_pages and
wp_list_categories has changed in the current 2.4-bleeding. I know
things in this are not set in stone so I'm hoping it's not a permanent change as it makes coding CSS menus that work with 2.3 and 2.4 very difficult especially as child item are no longer in a single container. The new structure also makes less sense semantically as each child is now a separate UL.

Changed from:

<ul>

<li>

Text
<ul>

<li>text</li>
<li>text</li>
<li>text</li>

<ul>

</li>

</ul>

To:

<ul>

<li>

Text
<ul>

<li>text</li>

<ul>
<ul>

<li>text</li>

<ul>
<ul>

<li>text</li>

<ul>

</li>

</ul>

(In [6835]) Walker markup fixes from hailin. fixes #5844

Looks like this is causing some problems with the depth arg.

(In [6839]) Revert [6835] as it breaks the depth arg. see#5844

new patch

the issue is that when $depth = 1, it creates empty <ul> </ul>.
The new patch addresses that: it checks earlier so it won't descend another level if not necessary. Verified with blogs with depth=1 theme (mwlibrary.wordpress.com).

(In [6858]) Walker markup fixes from hailin. fixes #5844