[BUGFIX] Setting userFuncClassPrefix to empty string does not work

Setting $GLOBALS['TYPO3_CONF_VARS']['FE']['userFuncClassPrefix']
to an empty string - to allow all prefixes - does not work as
expected as the check in t3lib_div expects a valid string with
length > 0. Adjust t3lib_div::hasValidClassPrefix to also
allow empty prefix var.

Change-Id: Idbd8cf7a547d9549f1c31dadd731eda48f52fcec
Resolves: #29133
Releases: 4.6
Reviewed-on: http://review.typo3.org/4445
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn