Simulator returns incorrect empty measurement record shapes when repetitions=0

[Prahalad-ship-it]

Describe the issue

When running a quantum circuit using Simulator().run() with repetitions=0, Cirq's sampling fallback path in SimulatesSamples.run_sweep_iter returns a hardcoded empty measurement array shape of (0, 1, 1).

This layout ignores the structural context of the circuit entirely. If a circuit contains multi-qubit measurement operations or repeated instances of the same measurement key, the returned placeholder dimension fails to match the actual layout criteria (repetitions, measurement_instances, measured_qubits), leading to incorrect downstream shape dimensions.

Explain how to reproduce the bug or problem

Execute the following minimal sample script:

import cirq

q0, q1 = cirq.LineQubit.range(2)

circuit = cirq.Circuit(
    cirq.measure(q0, q1, key="m"),
    cirq.measure(q0, q1, key="m"),
)

result = cirq.Simulator().run(circuit, repetitions=0)
print("Returned shape:", result.records["m"].shape)
# Expected: (0, 2, 2)
# Actual: (0, 1, 1)

Click to view failing assertion example log

AssertionError: Pattern dimension mismatch. 
Expected shape layout: (0, 2, 2)
Received placeholder array layout: (0, 1, 1)

Tell us the version of Cirq where this happens

# Paste the output of your version command here (e.g., 1.4.0.dev or similar)