[BUGFIX] Correct check for extTables script

Existence of the script defined in TYPO3_extTableDef_script is
currently achieved by using file_exists(). This is insufficient
if TYPO3_extTableDef_script is defined, but empty, as
file_exists() will match "typo3conf/" and try to include it.

Resolves: #46161
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/18828
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Change-Id: I9d6694228ca27cb2c446629a90e5c8636705e3e3
Reviewed-on: https://review.typo3.org/18829