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.
closesodoo/odoo#144222
Related: odoo/design-themes#751
Signed-off-by: Romain Derie (rde) <rde@odoo.com>