CODE QUALITY: Type annotations use None defaults without Optional

[Manideep3969]

Issue

Several dataclass fields default to None but are typed as the non-optional type:

• fusion.py: FusionResult fields original_circuit, optimized_circuit, etc.
• scheduling.py: ScheduleResult.circuit: QuantumCircuit = None
• Users accessing result.original_circuit.num_qubits on a default result get AttributeError

Fix

Use QuantumCircuit | None = None for these fields and add null-checks in property methods.