Default preferences are not displayed properly when using xwiki.properties

We can see an example here with the underline links preference: the user has underlining OFF (the link in the tips panel is not underlined). However it's shown as being the default `OnlyInlineLinks`. To reproduce, we remove the preference from the XWikiPreference object, and we add `underline=No` in the xwiki.properties file.

This UI is generated by https://github.com/xwiki/xwiki-platform/blob/f67730ee28e51fc8f412b24bc48f6d6e5e7c6ffa/xwiki-platform-core/xwiki-platform-user/xwiki-platform-user-profile/xwiki-platform-user-profile-ui/src/main/resources/XWiki/XWikiUserPreferencesSheet.xml#L294 .

After looking at it for a bit, here's our udnerstanding so far shared on the live chat:

> we use $xwiki.getXWikiPreference instead of xwiki.getUserPreference which in this case returns "" instead of the xwiki.properties value, and the hard coded fallback is not in line with the xwiki.properties value so it ends up as a bug.

Lucas C.

> so they're not the same
> getXWikiPreferences only checks in the XWiki.XWikiPreferences page and xobject of the same name

Vincent M.