[BUGFIX] RTE: JS exception regarding property disableStyleOnOptionLabel

Problem: BlockStyle plugin checks for property
this.pageTSconfiguration.disableStyleOnOptionLabel without checking
first that this.pageTSconfiguration exists at all.

Releases: master. 6.2
Resolves: #59574
Change-Id: Ic2554e44e6212475826d649416377f6c307df8db
Reviewed-on: http://review.typo3.org/33916
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>