test: add test for lammps null type

Signed-off-by: link89 <xuweihong.cn@gmail.com>

Author: link89

source/lmp/tests/test_lammps.py

@@ -517,6 +517,12 @@ def test_pair_deepmd_type_map(lammps_type_map) -> None:
     lammps_type_map.run(1)
 
 
+def test_pair_deepmd_type_map_with_null(lammps_type_map) -> None:
+    lammps_type_map.pair_style(f"deepmd {pb_file.resolve()}")
+    lammps_type_map.pair_coeff("* * H NULL")
+    lammps_type_map.run(0)
+
+
 def test_pair_deepmd_real(lammps_real) -> None:
     lammps_real.pair_style(f"deepmd {pb_file.resolve()}")
     lammps_real.pair_coeff("* *")