QE: Support output files without stress

[Yi-FanLi]

support qe output files which do not contain stress

[codecov[bot]]

Codecov Report

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

Comparison is base (d4b28ba) 82.85% compared to head (ff76af7) 82.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #572 +/- ## ========================================== + Coverage 82.85% 82.86% +0.01% ========================================== Files 74 74 Lines 6595 6601 +6 ========================================== + Hits 5464 5470 +6 Misses 1131 1131 ``` | [Files](https://app.codecov.io/gh/deepmodeling/dpdata/pull/572?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling) | Coverage Δ | | |---|---|---| | [dpdata/plugins/qe.py](https://app.codecov.io/gh/deepmodeling/dpdata/pull/572?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBkYXRhL3BsdWdpbnMvcWUucHk=) | `100.00% <100.00%> (ø)` | | | [dpdata/qe/scf.py](https://app.codecov.io/gh/deepmodeling/dpdata/pull/572?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepmodeling#diff-ZHBkYXRhL3FlL3NjZi5weQ==) | `94.40% <100.00%> (+0.18%)` | :arrow_up: |

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