[IMP] base: avoid invalidate cache on first assets

Assets bundle creation will try to find previous version of assets and unlink them.
This needs to invalidate ormcache to notify other workers.

Since assets are never commited during test, assets will be generate at each request if request is
not commited (cursor closed) whtch is the case with 404.

Invalidating the orm cache only when a previous attachement was existing will help to
mitigate the problem, and slightly improve HTTPCases test speed.

closes odoo/odoo#38815

Signed-off-by: Xavier Dollé (xdo) <xdo@odoo.com>