docstr: Test matrices list is provided, but empty. Operation should have `0` label and matrix
  should be added to cache.
docstr_tok: ["Test","matrices","list","is","provided",",","but","empty",".","Operation","should","have","`","0","`","label","and","matrix","should","be","added","to","cache","."]
code: def test_empty_cache_list(self, mat, op):
      """Test matrices list is provided, but empty. Operation should have `0` label and matrix
      should be added to cache."""
      cache = {"matrices": []}
      assert op.label(cache=cache) == "U(M0)"
      assert qml.math.allclose(cache["matrices"][0], mat)
  
imports: import pytest
  import numpy
  from scipy.stats import unitary_group
  from gate_data import TSWAP
  import pennylane
  from pennylane.wires import Wires
  from pennylane.operation import DecompositionUndefinedError
label_desc: Use image node_id 4 for calling the TestUnitaryLabels obj's underlying member method code with example usage: obj.test_empty_cache_list(mat, op) without return types