[BUGFIX] Test failures in cli context

Patch 1ad2cb85 for issue #39706 broke some iconWorks tests in
cli context. Solution is to make the initialize method of the sprite
manager a bit smarter. This allows a delayed initialization,
for example in unit tests that need it.

Change-Id: Ib1f85446cf5304b03c29e52e5be588ed12f9b0de
Resolves: 6.0
Releases: #39717
Related: #39706
Reviewed-on: http://review.typo3.org/13606
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn