fix(data_frame): parsing empty query result value as `numpy.NaN`

[bednar]

Closes #550

Proposed Changes

The empty values in query results from serialization into Pandas DataFrame are returned as numpy.NaN values instead of '' - empty string.

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] A test has been added if appropriate
• [x] pytest tests completes successfully
• [x] Commit messages are conventional
• [x] Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8fa5146) 90.29% compared to head (fb19fe9) 90.30%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #584 +/- ## ======================================= Coverage 90.29% 90.30% ======================================= Files 39 39 Lines 3452 3455 +3 ======================================= + Hits 3117 3120 +3 Misses 335 335 ``` | [Impacted Files](https://app.codecov.io/gh/influxdata/influxdb-client-python/pull/584?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=influxdata) | Coverage Δ | | |---|---|---| | [influxdb\_client/client/flux\_csv\_parser.py](https://app.codecov.io/gh/influxdata/influxdb-client-python/pull/584?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=influxdata#diff-aW5mbHV4ZGJfY2xpZW50L2NsaWVudC9mbHV4X2Nzdl9wYXJzZXIucHk=) | `98.42% <100.00%> (+0.01%)` | :arrow_up: |

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