fix vasp/xml errors when no virial is printed (#574)

Fix #212.

---------

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: njzjz

dpdata/plugins/vasp.py

@@ -109,7 +109,7 @@ def from_labeled_system(self, file_name, begin=0, step=1, **kwargs):
             data["coords"],
             data["energies"],
             data["forces"],
-            data["virials"],
+            tmp_virial,
         ) = dpdata.vasp.xml.analyze(
             file_name, type_idx_zero=True, begin=begin, step=step
         )
@@ -121,9 +121,11 @@ def from_labeled_system(self, file_name, begin=0, step=1, **kwargs):
         for ii in range(data["cells"].shape[0]):
             data["coords"][ii] = np.matmul(data["coords"][ii], data["cells"][ii])
         # scale virial to the unit of eV
-        v_pref = 1 * 1e3 / 1.602176621e6
-        for ii in range(data["cells"].shape[0]):
-            vol = np.linalg.det(np.reshape(data["cells"][ii], [3, 3]))
-            data["virials"][ii] *= v_pref * vol
+        if tmp_virial.size > 0:
+            data["virials"] = tmp_virial
+            v_pref = 1 * 1e3 / 1.602176621e6
+            for ii in range(data["cells"].shape[0]):
+                vol = np.linalg.det(np.reshape(data["cells"][ii], [3, 3]))
+                data["virials"][ii] *= v_pref * vol
         data = uniq_atom_names(data)
         return data