[FIX] categories: Clear the $tree var if category is empty

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@49662 b456876b-0849-0410-b77d-98878d47e9d5
[bp/r49068]Usability FIX, in fact, for all profiles from the Profiles Wizard:'[ENH]When installing a Featured Site Profile from the Config Wizards page, you don't need to choose the same profile twice anymore'. Thanks alain_desilets

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@49664 b456876b-0849-0410-b77d-98878d47e9d5