[BUGFIX] Additional extTables script might not exist

During https://review.typo3.org/17381, the constant
TYPO3_extTableDef_script is now always defined.

Change the bootstrap to only include the file if it really
exists.

Related: #44331
Fixes: #45122
Releases: 6.0, 6.1
Change-Id: I547c5e141b2395eb8b684ce245bb0440096f14df
Reviewed-on: https://review.typo3.org/17994
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn