[Misc] "null" doesn't exist in Velocity, use a non-existing $NULL variable instead to represent null

XWIKI-9924: Inconsistent font size and font weight in the tag cloud container header

[misc] Fully fixed any remaining limitation introduced by previous performance improvement (even if it was for pretty rare use cases)

* Cleaning
** removing extension.flavor
** rolling back extension.defaultui.wiki that was badly updated instead of extension.flavor.wiki
** upgraded extension.flavor.wiki

* Fix the broken test.

* Disable the flavor picker on DW for subwikis when a default UI is set.