[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#108017

X-original-commit: 9bdd8faea9f0126305856a40b3215589e6b3e610
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>

addons/web_editor/static/src/js/editor/snippets.options.js

@@ -6991,7 +6991,12 @@ registry.BackgroundShape = SnippetOptionWidget.extend({
                 shapeToSelect = possibleShapes.find((shape, i) => {
                     return possibleShapes[i - 1] === previousShape;
                 });
-            } else {
+            }
+            // If there is no previous sibling, if the previous sibling had the
+            // last shape selected or if the previous shape could not be found
+            // in the possible shapes, default to the first shape. ([0] being no
+            // shapes selected.)
+            if (!shapeToSelect) {
                 shapeToSelect = possibleShapes[1];
             }
             this.trigger_up('snippet_edition_request', {exec: () => {