Fix "ValueError: The truth value of an empty array is ambiguous. Use …

…`array.size > 0` to check that an array is not empty."

Signed-off-by: Jinzhe Zeng <[email protected]>

tests/test_abacus_pw_scf.py

@@ -158,7 +158,7 @@ def test_noforcestress_job(self):
         # check below will not throw error
         system_ch4 = dpdata.LabeledSystem("abacus.scf", fmt="abacus/scf")
         # check the returned force is empty
-        self.assertFalse(system_ch4.data["forces"])
+        self.assertFalse(system_ch4.data["forces"].size)
         self.assertTrue("virials" not in system_ch4.data)
         # test append self
         system_ch4.append(system_ch4)