[BUGFIX] BE login form gives warnings in RteHtmlParser

Intialize configuration as array to prevent PHP warnings for
Illegal string offset.

Change-Id: Ic7398ba2c041c3278a1bd5562968ac5183643d3d
Fixes: #43291
Releases: 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/19705
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring