[Backport IE fixes to 1.7] Fix JS errors occurring during EnterKeyHandling in IE11. Curiously, the tests did not catch them. Fixes #17466.

Note: I am not adding the has("trident") test that was added in other branches because I can't seem to reproduce on IE11...

(cherry picked from commit 8ec9162b32398ed7a2a15272e277532698a6512d)

Conflicts:
	_editor/RichText.js
	_editor/plugins/EnterKeyHandling.js

(cherry picked from commit 1e0d8c0945335796f09fce4d4eeb9105c28b4b69)

Conflicts:
	_editor/RichText.js