[BUGFIX] Warnings in tcemain on iterating over unset columns

Adds a check if the TCA for the table has a columns setting at all
before iterating over the columns of that table.

Change-Id: Id5f968d5c6b6df0df5285de1311c537089570bce
Fixes: #32467
Branches: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/9061
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer