[FIX] web_editor: fix picking a shape when sibling has the last one

Commit [1] introduced the shape system. This system was able to chain
shapes depending on the shape of the previous sibling element.

Unfortunately, if the shape selected on the sibling was the last shape
available, clicking on the toggle shape button would result in nothing
happening.

This commit fixes that by defaulting to the first shape if no possible
shapes are given by the sibling.

[1]: https://github.com/odoo/odoo/commit/b84e0af742c51b88b4c108ebec2d0c7fff4b7483



opw-3082292

closes odoo/odoo#108000

X-original-commit: 9bdd8fae
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>