[FIX] base: fix asset generation when there is .less files

Since commit f7130556bb the compiled assets would be returned in a loop
before executing the rest of the `preprocess_css` method.
But every loop would override the previous one as it is `=` and not `+=`.

Fixes #27922

closes odoo/odoo#29109

odoo/addons/base/models/assetsbundle.py

@@ -439,7 +439,7 @@ class AssetsBundle(object):
                 assets = [asset for asset in self.stylesheets if isinstance(asset, atype)]
                 if assets:
                     source = '\n'.join([asset.get_source() for asset in assets])
-                    compiled = self.compile_css(assets[0].compile, source)
+                    compiled += self.compile_css(assets[0].compile, source)
 
             # We want to run rtlcss on normal css, so merge it in compiled
             if self.user_direction == 'rtl':