MAHOUT-608: handle 0-qubit circuits in cirq backend to prevent crash

[shiavm006]

Purpose

Fixes crash in Cirq backend when executing 0-qubit circuits with measurements.

Related Issues

closes #608

Changes Made

• Added guard in execute_circuit() to handle empty circuits before adding measurements
• Returns Cirq-formatted result [{0: shots}] for 0-qubit case
• Prevents crash when cirq.measure() is called on empty qubit list

Technical Details

The issue occurred at line 106 where circuit.append(cirq.measure(*circuit.all_qubits(), key="result")) crashes on empty circuits because circuit.all_qubits() returns an empty list.

This fix aligns with existing behavior in the main QuMat class (lines 92-97 in qumat.py) which already handles 0-qubit circuits. Moving this logic to the backend makes it self-contained and robust.

Breaking Changes

None. This is a bug fix that maintains existing behavior.

Checklist

• Code follows project style guidelines
• Changes are minimal and focused on the issue
• Implementation aligns with maintainer feedback
• No breaking changes introduced

[guan404ming]

LGTM, thanks!