CSV IO: don't crash on empty CSV

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (bbf6c71) 95.58% compared to head (43e495b) 95.58%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #178 +/- ## ======================================= Coverage 95.58% 95.58% ======================================= Files 38 38 Lines 2491 2492 +1 Branches 470 471 +1 ======================================= + Hits 2381 2382 +1 Misses 86 86 Partials 24 24 ``` | [Impacted Files](https://codecov.io/gh/BEMServer/bemserver-core/pull/178?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer) | Coverage Δ | | |---|---|---| | [bemserver\_core/input\_output/sites\_io.py](https://codecov.io/gh/BEMServer/bemserver-core/pull/178?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvaW5wdXRfb3V0cHV0L3NpdGVzX2lvLnB5) | `100.00% <ø> (ø)` | | | [bemserver\_core/input\_output/base.py](https://codecov.io/gh/BEMServer/bemserver-core/pull/178?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer#diff-YmVtc2VydmVyX2NvcmUvaW5wdXRfb3V0cHV0L2Jhc2UucHk=) | `96.77% <100.00%> (+0.10%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BEMServer)

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