Computed field marked as optional should generate a nullable type

[Azzerty23]

Description and expected behavior
A computed field marked as optional should generate a "nullable type", such as OperandExpression<number | null>, instead of OperandExpression<number>.

Environment (please complete the following information):

• ZenStack version: [e.g., 3.0.0-alpha.4]

Additional context
See agePlus2 computed field in the generated schema.ts and the corresponding typing issue here.