Allow Thermostat and Zone to exist without having a setpoint

[climategadgets]

Existing Behavior

Control logic is dead set on having a setpoint from the get go and refuses to work without it. This forces some value of a setpoint to be injected at instantiation, and "default" reasonable setpoint value is different for different zones and different modes, hence, impractical.

Desired Behavior

Allow setpoint to be unspecified at startup so that the control logic can interpret it exactly as it does when the signal source for that zone is missing.

Additional Context

Same applies to the economizer - as long as permanent configuration is provided, missing changeoverDelta and target may be interpreted as "keep the economizer off", and missing keepHvacOn may be configured to a sensible default.