Empty string in database becomes None value in Python

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### Specifications

* Client Version: 1.34.0
* InfluxDB Version: 2.6.1
* Platform: Linux

### Code sample to reproduce problem

```python
import influxdb_client

with influxdb_client.InfluxDBClient(
org = ...,
token = ...,
url = ...,
) as client:
q = f"""
from(bucket: ...)
//|> filter(fn: (r) => exists r._value)
//|> filter(fn: (r) => r._value != "")
|> last()
"""
res = [
record_it.values
for table_it in client.query_api().query(q)
for record_it in table_it.records
]
print(res)
```

### Expected behavior

[{'result': '_result', 'table': 0, '_time': datetime.datetime(2023, 3, 2, 18, 47, 7, 547036, tzinfo=tzutc()), '_value': ''}]

### Actual behavior

[{'result': '_result', 'table': 0, '_time': datetime.datetime(2023, 3, 2, 18, 47, 7, 547036, tzinfo=tzutc()), '_value': None}]

### Additional info

[`last()` function](https://docs.influxdata.com/flux/v0.x/stdlib/universe/last/):

> If \[`_value`\] column is null in the last record, last() returns the previous record with a non-null value.

My last record with a non-null `_value` has the empty string for `_value`.
So far, the row is correctly identified.
But by the time I read `_value` from the result returned in Python, the record does not hold the empty string `""` but `None`.

I verified that the value of `_value` is really the empty string in the database and not `None` by experimenting with filters (see commented code).
I find it more intuitive to leave the empty string as empty string in Python.

Start by running the Python reproduction with the shown Flux query and compare the expected and actual _value values. Trace how query_api().query(q) parses the returned record, then verify that an empty string remains "" rather than becoming None in the result.

Written by the indexing model from the issue text.