[FIX] web_editor: fix line merge on color change

Before this commit:

When using shift+enter to move to a new line, now applying bold, italics,
underline, etc and then trying to add a color to both lines would result in
lines merging.

After this commit:

Now when the both lines are bold and then applying a color to both the lines
would no longer merge.

closes odoo/odoo#118698

Task: 3269922
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>