[FIX] website: don't interpolate null values for css properties

In 29f02a37f0 we bumped the verion of libsass that we use. One
breaking change introduced by the new version is that if a css variable
is interpolated to the empty string, it is still emited in the compiled
css [1]

This commit fixes that by explicitly omitting null values from the map
before looping over it.

[1]: https://sass-lang.com/documentation/breaking-changes/css-vars

task-2248360

closes odoo/odoo#50668

X-original-commit: 6faf327706169bbe31e188a42805085b95b78052
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/website/static/src/scss/website.scss

@@ -37,7 +37,7 @@ $-font-numbers: (
     // 3) The values in the $colors map are also printed by Bootstrap.
 
     // 4) The Odoo values map, $o-website-values, must be printed.
-    @each $key, $value in $o-website-values {
+    @each $key, $value in o-map-omit($o-website-values) {
         --#{$key}: #{$value};
     }