[BUGFIX] Warning in addPlugin()

ExtensionManagementUtility::addPlugin() may raise a warning if
a subarray of TYPO3_LOADED_EXT is not set for whatever reason.
The patch sanitizes this.

Change-Id: I4fb8c24bcdebd6b04e856ecad5da07e624d95da4
Resolves: #60122
Related: #59770
Releases: 6.3, 6.2
Reviewed-on: https://review.typo3.org/31357
Reviewed-by: Marcin Sągol
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring