Implement nullability over scalars

[javihern98]

Summary

Currently, the datastructures of some components may differ between semantic execution and full execution.

This is due to how nullability is calculated in operations such as dataset[calc measure := scalar | constant]. This is because of how nullability is calculated in some RoleSetter operators. For example, in the Mesure operator, among the various checks, it is verified that the value of scalar | constant is not None.

if cls.role == Role.IDENTIFIER or operand.value is not None:
     nullable = False

However, in the validate checks, it is always set to None, and in the evaluate checks, a different value can be assigned to the Scalar, causing the nullability to differ between executions.

A nullability must be implemented internally in the Scalars model so that the structure and nullability are carried over correctly.

The nullability of a Scalar input will be taken as True by default, and that of Constants as False (except for explicitly Null Constants).

[javihern98]

@mla2001 Setting this P0 after our discussion 😊