[TASK] Improve rootline cache handling

The method get() checks if there is an entry by using
has() and does then an additional get(). This can
be simplified which saves a query for every hit

Change-Id: I0ebfd8c7622a5e673d3f34aa685f18a9878116ef
Resolves: #45530
Releases: 6.1
Reviewed-on: https://review.typo3.org/18314
Reviewed-by: Oliver Hader
Reviewed-by: Philipp Gampe
Reviewed-by: Steffen Ritter
Reviewed-by: Oliver Klee
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn