QPY serialization of empty gates yields ``None`` definitions

Environment

Qiskit Terra version: 0.19.1
Python version: 3.9
Operating system: macOS

What is happening?

Serializing + deserializing a circuit containing an empty gate (i.e. no operations) with QPY yields a gate where the definition is None instead of an empty circuit. If the definition is None the transpiler cannot decompose the gate, unless the gate is in the basis set (like e.g. x or cx are).

This is a problem in particular for Qiskit runtime as users might unintentionally construct circuits that contain empty gates, as e.g. was the case in Qiskit/qiskit-nature#449.

How can we reproduce the issue?

import io
from qiskit import Aer, transpile, QuantumCircuit
from qiskit.circuit import QuantumCircuit
from qiskit.circuit.qpy_serialization import dump, load

empty = QuantumCircuit(1, name="empty")
circuit = QuantumCircuit(1)
circuit.append(empty, [0])

qpy_file = io.BytesIO()
dump(circuit, qpy_file)
qpy_file.seek(0)
new_circuit = load(qpy_file)[0]

sim = Aer.get_backend("aer_simulator")
transpile(new_circuit, sim)

produces

qiskit.exceptions.QiskitError: "Cannot unroll the circuit to the given basis, ['ccx', 'cp', 'cswap', 'csx', 'cu', 'cu1', 'cu2', 'cu3', 'cx', 'cy', 'cz', 'delay', 'diagonal', 'h', 'id', 'initialize', 'mcp', 'mcphase', 'mcr', 'mcrx', 'mcry', 'mcrz', 'mcswap', 'mcsx', 'mcu', 'mcu1', 'mcu2', 'mcu3', 'mcx', 'mcx_gray', 'mcy', 'mcz', 'multiplexer', 'p', 'pauli', 'r', 'roerror', 'rx', 'rxx', 'ry', 'ryy', 'rz', 'rzx', 'rzz', 's', 'sdg', 'swap', 'sx', 'sxdg', 't', 'tdg', 'u', 'u1', 'u2', 'u3', 'unitary', 'x', 'y', 'z', 'kraus', 'roerror', 'save_amplitudes', 'save_amplitudes_sq', 'save_density_matrix', 'save_expval', 'save_expval_var', 'save_matrix_product_state', 'save_probabilities', 'save_probabilities_dict', 'save_stabilizer', 'save_state', 'save_statevector', 'save_statevector_dict', 'save_superop', 'save_unitary', 'set_density_matrix', 'set_matrix_product_state', 'set_stabilizer', 'set_statevector', 'set_superop', 'set_unitary', 'snapshot', 'superop']. Instruction empty not found in equivalence library and no rule found to expand."

What should happen?

The gate should have a definition which is an empty circuit instead of being just None.

Any suggestions?

No response

Qiskit / qiskit