XWIKI-11041: When {{xwiki.defaultbaseskin}} is empty, XWiki#getDefaultBaseSkin() returns {{xwiki.defaultskin}}.
* some optimizations

(cherry picked from commit db0ecaf0c4c9f6486eb0a59a36569d5b9d448f10)
XWIKI-11042: Add direct Collection#getValue API
[performances] Remove useless calls to Object.get

(cherry picked from commit 7a3ab61babf3dc52a1df3097db4a9b5915402c19)