[BUGFIX] RTE: JS error raised in Abbreviation dialogue

When no abbreviation and/or acronym is pre-defined, hitting the ok
button of the dialogue raises a js error.

Releases: master, 6.2
Resolves: #63030
Change-Id: I4df0b3896a53808ef2bd8ef6ee8ba71da7030735
Reviewed-on: http://review.typo3.org/34267
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>