Fix turning off preformatted text applying a different style than default in the editor

[paw-hub]

Passing null to setFontFace will remove the font-face attribute in the editor. Therefore, if we do this for turning off preformatted text, it will remove the preformatted text.

Before, we were setting to sans-serif which is generally a different font than the default font for the editor.

Fixes #4103