[FIX] website_payment: fix "Donation" snippet "Pre-Filled Options"

Steps to Reproduce :

- Install eCommerce.
- Drag and drop the "Donation" snippet.
- Click on the prices in the snippet.
- Toggle the "Pre-Filled Options" option.
- => There is a traceback.

This bug is due to the fact that since commit [1], all templates have
been added to the OWL app. With OWL, a t-foreach in a template can no
longer loop over a 0 number and causes an error if we try to. This is
what happens here, as toggling off the "Pre-filled Options" option sets
the donationAmounts to 0, which is then used in a loop in the templates.

This commit fixes this by replacing the number by an empty array, so the
loop cannot fail.

[1]: https://github.com/odoo/odoo/commit/4703e4a2efa9213979307e4d3dedeedc61ad0fc3

task-3859207

closes odoo/odoo#161431

Signed-off-by: Soukéina Bojabza (sobo) <sobo@odoo.com>

addons/website_payment/static/src/snippets/s_donation/options.js

@@ -289,7 +289,7 @@ options.registry.Donation = options.Class.extend({
                 this.$target.find('.s_donation_donate_btn').before(sliderTemplate);
             }
             const prefilledOptions = this.$target[0].dataset.prefilledOptions;
-            let donationAmounts = 0;
+            let donationAmounts = [];
             let showDescriptions = false;
             if (prefilledOptions) {
                 donationAmounts = JSON.parse(this.$target[0].dataset.donationAmounts);