[BUGFIX] Test typeof TBE_EDITOR for object not function

As TBE_EDITOR is defined as object in jsfunc.tbe_editors.js we should
change the check here. Otherwise no eval user functions will be called.

Resolves: #57296
Releases: 6.2, 6.1
Change-Id: Ie1b701dcbf465827bc336233c2523f4df1dde9ba
Reviewed-on: https://review.typo3.org/28810
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes