[FIX] web: fallback on basic aggregate computation when currency is absent

Since [0], a currency field is required to be in the view in order to
compute the aggregate values in a list view. As it turns out, many views
in odoo ([1], [2], ...) didn't meet this requirement. In stable, views
are not updated automatically and it requires a manual intervention.

Even though the views should still be fixed, this commits allows falling
back on the default number of digits in case the currency is missing.

[0]: https://github.com/odoo/odoo/commit/f112e923ff18679b4bf546677f240f6bf83f3bb2
[1]: https://github.com/odoo/odoo/commit/eaec9f7c3c28e2ee9d42597052a21e6b2695cd60
[2]: https://github.com/odoo/enterprise/commit/5ca99702e44ef09aed504e93a302bfe43997193e

opw-3316448

Part-of: odoo/odoo#125892