[BUGFIX] Incomplete mock raises PHP 5.4 warning

Fix the travis fail on PHP 5.4, introduced by #45254

Related: #45254
Resolves: #47449
Releases: 6.1, 6.0

Change-Id: I4f4f4df62989ad67c3b32aea304a8718f0c344b2
Reviewed-on: https://review.typo3.org/20079
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn