[BUGFIX] Retrieving extension fails with some PHP versions

XmlParser has an issue with PHP < 5.4.28 leading to
unexpected empty arrays raising warnings. If development
preset is activated, warnings are turned into exceptions,
so the extension list parser stops importing.

Resolves: #58418
Releases: 6.2, 6.1
Change-Id: Idc6453bd8dcc46a933a1d6d72361ffff5842e39d
Reviewed-on: https://review.typo3.org/29783
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring