[BUGFIX] Provide TSFE in v:format.case in BE

BE doesn't provide $GLOBALS['TSFE']->csConvObj and format caseViewhelper relays on it.
Solution from fluid crop https://github.com/NamelessCoder/fluid/blob/master/TYPO3/Fluid/ViewHelpers/Format/CropViewHelper.php
`if ('BE' === TYPO3_MODE) {` (also below)