GMSO failing to create an empty box for MC dual simulation box setups

**Bug summary** GMSO not allowing empty boxes to be created for MC simulations with dual boxes in 0.15.0, which is permitted in 0.14.0. See new failing tests in mosdef-gomc (https://github.com/GOMC-WSU/MoSDeF-GOMC/actions/runs/19054940022) **Code to reproduce the behavior** Build mosdef GOMC and run the tests on 0.15.0 and in 0.14.0. There are 3/4 tests that fail because of this - def test_box_1_empty_test_1 - def test_box_1_empty_test_2 - def test_box_1_empty_test_3 The 4th failing test "test_save_charmm_gomc_ua_charmm_periodic_wildcard_improper_ff", is not an issue an just some change that can be fixed by changing the order of the testing file for the 2 center (both wildcard) atoms in the improper. Please include a code snippet that can be used to reproduce this bug. PLEASE USE THE MOSDEF-GOMC tests! I have also provided a simpler version below, but it may not contain all possibilities. from foyer.forcefields import forcefields from gmso import Topology from gmso.exceptions import GMSOError from gmso.external.convert_mbuild import from_mbuild, to_mbuild from mbuild import Box, Compound from mbuild.lattice import load_cif from mbuild.utils.io import get_fn, has_foyer from mosdef_gomc.formats.gmso_charmm_writer import ( Charmm, _Exp6_Rmin_to_sigma, _Exp6_Rmin_to_sigma_solver, _Exp6_sigma_to_Rmin, _Exp6_sigma_to_Rmin_solver, ) from mosdef_gomc.tests.base_test import BaseTest from mosdef_gomc.utils.conversion import ( base10_to_base16_alph_num, base10_to_base22_alph, base10_to_base26_alph, base10_to_base44_alph, base10_to_base52_alph, base10_to_base54_alph_num, base10_to_base62_alph_num, ) from mosdef_gomc.utils.io import get_mosdef_gomc_fn two_propanol_ua = mb.load('CC(C)O', smiles=True) two_propanol_ua.name = 'POL' empty_compound = Box(lengths=[2, 2, 2]) box_0 = mb.fill_box( compound=[two_propanol_ua], n_compounds=[1], box=[4, 4, 4] ) print(f'box_0= {box_0}') charmm = Charmm( box_0, "charmm_filled_box", structure_box_1=empty_compound, filename_box_1="charmm_empty_box", ff_filename="charmm_empty_box.inp", residues=[two_propanol_ua.name], forcefield_selection="oplsaa", bead_to_atom_name_dict={"_CH3": "C"}, atom_type_naming_style="general", ) charmm.write_pdb() charmm.write_psf() ``` **Software versions** - Which version of GMSO are you using? 0.15.0 does not work... 0.14.0 does work - Which version of Python (`python --version`)? 3.12.12 - Which operating system? Ubuntu 24.04