[FIX] web_editor: prevent crash when colorpickers receive split values

Steps to reproduce:
- Edit
- Add a snippet with columns
- Save
- HTML editor
- Set up a border on one column of the snippet with different colors for
  the top/right/bottom/left sides
- Edit
- Click on the column => crash

This is only an example, but if a colorpicker widget is linked to a
SnippetOption instance's `selectStyle` method designed to handle the
"border-color" property of an element, the value received can be split
if the item uses different colors for its top/right/bottom/left borders.
For instance, you could receive "red blue" if the item has red top and
bottom borders and blue left and right borders. In that case, the
colorpicker widget code would try to add the class "bg-red blue" on its
preview item which would crash because of the space inside).

After this commit, we simply do not show any color for this case.

opw-2803311

closes odoo/odoo#103654

Signed-off-by: Romain Derie (rde) <rde@odoo.com>