fix vasp/xml errors when no virial is printed

[njzjz]

Fix #212.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e52b699) 82.87% compared to head (e636643) 82.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #574 +/- ## ======================================= Coverage 82.87% 82.88% ======================================= Files 74 74 Lines 6606 6608 +2 ======================================= + Hits 5475 5477 +2 Misses 1131 1131 ``` | [Files](https://app.codecov.io/gh/deepmodeling/dpdata/pull/574?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpdata/plugins/vasp.py](https://app.codecov.io/gh/deepmodeling/dpdata/pull/574?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBkYXRhL3BsdWdpbnMvdmFzcC5weQ==) | `100.00% <100.00%> (ø)` | |

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