[BUGFIX] fix bug in storing the useCache property

This was not stored correctly if the extension configuration
was missing.
Now it uses the default (false) if no configuration is found.

Change-Id: I3d7308f382ad51d156041d1c4f769ee87c1fef28
Reviewed-on: https://review.typo3.org/31563
Reviewed-by: Christian Zenker
Tested-by: Christian Zenker