bool() always return True on cartesian_product

[user202729]

Steps To Reproduce

sage: cartesian_product([Set([])])
The Cartesian product of ({},)
sage: cartesian_product([Set([])]).is_empty()
True
sage: bool(cartesian_product([Set([])]))
True

Expected Behavior

bool should return not self.is_empty()

Actual Behavior

it always return True

Additional Information

Basically this is caused by the __bool__ implementation of Parent. The is_empty is in Sets.CartesianProducts.ParentMethods, but putting another __bool__ there cannot override because it is below.

Environment

• OS: Linux
• Sage Version: latest master

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide