Allow QuantumCircuit construction with zero qubit or clbit count

[eendebakpt]

Summary

This PR allows construction of circuits with zero registers (either quantum or classical). The construction of these circuits is convenient with algorithmically constructing circuits with varying numbers of registers.

This is a variation on #6506. For circuits with zero qubits no quantum registers are created.

from qiskit.circuit import QuantumCircuit
for q in range(0,2):
    for c in range(0,2):
        circ = QuantumCircuit(q,c)
        print(f'QuantumCircuit({q},{c}): {circ.qregs} {circ.cregs}') 

circ = QuantumCircuit(0)
print(f'QuantumCircuit(0): {circ.qregs} {circ.cregs}') 

circ = QuantumCircuit(0)
QuantumCircuit(0,0): [] []
QuantumCircuit(0,1): [] [ClassicalRegister(1, 'c')]
QuantumCircuit(1,0): [QuantumRegister(1, 'q')] []
QuantumCircuit(1,1): [QuantumRegister(1, 'q')] [ClassicalRegister(1, 'c')]
QuantumCircuit(0): [] []

Details and comments

Variation is based on comments from #6506

@kdk

[kdk]

Thanks for the updates @eendebakpt , this looks good!