Add support for empty rules in graph-decomposition

[maliasadi]

Context:
Add support for empty rules in graph-decomposition.

Description of the Change:

Benefits:

import pennylane as qp

from catalyst import qjit
from catalyst.jax_primitives import decomposition_rule
from catalyst.passes import graph_decomposition


@decomposition_rule(op_type=qp.PauliX)
def x_to_rx(_wire: int):
    pass

@qjit(capture=True)
@graph_decomposition(
    gate_set={qp.RX, qp.RY},
    fixed_decomps={qp.PauliX: x_to_rx},
)
@qp.qnode(qp.device("lightning.qubit", wires=2))
def circuit():
    qp.PauliX(0)
    qp.PauliX(1)
    qp.RX(0.5, wires=0)

    # register custom decomposition rules
    x_to_rx(int)

    return qp.state()

qp.specs(circuit, level="device")().resources.gate_types

{'RX': 1}

Related GitHub Issues:
[sc-116539]

[kipawaa]

Great addition, thank you!

[astralcai]

Looks straightforward enough, thanks!