RZZ gate in hqslib1 QASM translated to CustomGate with empty circuit

[cqc-alec]

from pytket.qasm import circuit_from_qasm_str, circuit_to_qasm_str
from pytket.passes import DecomposeBoxes

c = circuit_from_qasm_str(
    """
OPENQASM 2.0;
include "hqslib1.inc";

qreg q[2];
RZZ(0.5*pi) q[0],q[1];
"""
)

DecomposeBoxes().apply(c)

print(circuit_to_qasm_str(c, header="hqslib1"))

Output:

OPENQASM 2.0;
include "hqslib1.inc";

qreg q[2];

[cqc-alec]

Fixed by #679 .