[Full-stack Signals] make default value optional

[platosha]

Calling a signal endpoint requires providing initial client-side value. Coming up with initial value for complex types is inherently complex. A full value could end up as a lengthy literal:

const person = new PersonEndpoint.personSignal({defaultValue: {name: '', dob: '1970-01-01'}});

This complexity could be mitigated with some help from a generated model:

const person = new PersonEndpoint.personSignal({defaultValue: PersonModel.createEmptyValue()});

However, we could reduce the complexity even further by using the model data on the Hilla end, so that using the signals don't require default value:

const person = new PersonEndpoint.personSignal(); // initialized using the model

[Legioth]

From where does 1970-01-01 end up as the date of birth in PersonModel.createEmptyValue()?

[platosha]

For nullable signals, we could use undefined as the default value instead of the model's value.