[BUGFIX] Cast autoload and classAliasMap to Array

If you create an empty ext_autoload.php or an empty classAliasMap.php
file or fail to return an array, NULL is merged with the existing
array which results in NULL for the array $classRegistry or
$aliasToClassNameMapping
This patch casts the required Array(which is possibly NULL) to an
Array to prevent array_merge with NULL values.

Fixes: #51474
Releases: 6.2, 6.1, 6.0
Change-Id: I1887da3a942b17e441338a95cb7da49a17535907
Reviewed-on: https://review.typo3.org/23441
Reviewed-by: Kai Ole Hartwig
Tested-by: Kai Ole Hartwig
Reviewed-by: Christoph Dörfel
Tested-by: Christoph Dörfel
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn