[FIX] base: prevent "learn more" to be shown when no website/link

Apps have a "learn more" button which is redirecting to the given
website/URL.
The website is a field of the `ir.module.module` record, typically set
on enteprise apps in this file [1].

But the apps without a website are incorrectly showing that button.
Clicking on it will redirect the user to a `/false` 404 page, except in
16.0 and later version where it will simply do nothing.

Seems like there was an error introduced at some point, I didn't dig
into a complete history check but [2] could be a start.

[1]: https://github.com/odoo/odoo/blame/11d023f068d0308f2fa2de45f932f61a95086c6b/odoo/addons/base/data/ir_module_module.xml#L16
[2]: https://github.com/odoo/odoo/commit/7f9e7f0c961106fe13419166f112007f2d04e6ac#diff-c6e4af323e7ca3fe9d069fa98cd5da2f18ba606b8e4262005a79d2c1e0890dc8R189



closes odoo/odoo#106985

X-original-commit: a9905518
Signed-off-by: Romain Derie (rde) <rde@odoo.com>