Update stub-builder.ts to support falsy overrides

[darcyrush]

The current override logic checks if an override property exists by using falsy logic to check its value, so if an override defined with a value of false or 0, the override will instead be mocked.

Using the in operator checks if the property exists rather than checking the value of the property, allowing these overrides

[ShellyDCMS]

thank you @darcyrush !