[BUGFIX] LocalConfiguration extListArray should be written without keys

When writing LocalConfiguration.php, arrays should be written without keys
if all the keys are numeric, even if the keys contain leaps.

Fixes: #44938
Releases: 6.1, 6.0
Change-Id: Ic1d929d54a75b091ecbe4f0f4b82049d507ccd95
Reviewed-on: https://review.typo3.org/17811
Reviewed-by: Mattias Nilsson
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn