[8.0][FIX][server] onchange method returns too many fields and may crash...

... the safe_eval() method (with a ValueError: name 'fx' is not defined while evaluating
[f1, f2, ...]) when implying "cascading" onchange on some non expected fields
This screws up the recomputation of computed fields.
There are two reasons for a field to not be in cache:

* the field is not present in the view, hence no value is provided
* the field has been invalidated (for computed fields)

You address the first case correctly, but in the second one you have to evaluate the field.