A few checks in different places didn't account for zero-operand gates
being possible.  In most uses, there never would be anything useful
here, but it can occasionally be useful to create a "global-phase"
instruction, especially if this is later going to have controls applied
to it.  This is well-formed within the Qiskit data model; spectator
qubits implicitly undergo the identity, so this corresponds to the
global phase being multiplied by the identity on _all_ qubits.

…ands

…ands