During upgrades, customization modules are defined in the database but their codebase is not in the addons path. Because of this, when running an upgrade the manifest of such modules does not exist which makes `_generate_primary_snippet_templates` fail because it cannot locate some default keys. This commit adapts the access to the manifest in order to use a default value in case the manifest is not available. closes odoo/odoo#144222 Related: odoo/design-themes#751 Signed-off-by: Romain Derie (rde) <rde@odoo.com>