InfluxQLQueryApiImpl cannot cast JsonNull

@NguyenHoangSon96 ci sta già lavorando.

Dal 3/2/2025.

Hi,
I'm trying to use `queryJSON` method of `InfluxQLQueryApi`.
If I have `null` values resulting from my query, the library fires an exception because of the cast forced to String.

As we can see from line 337 of the class:

```
JsonArray jvals = sobj.get("values").getAsJsonArray();
if (jvals != null) {
for(JsonElement jval : jvals) {
List values = new ArrayList();
JsonArray jae = jval.getAsJsonArray();
int index = 0;

for(JsonElement je : jae) {
List columnKeys = new ArrayList(serie.getColumns().keySet());
if (this.extractor != null) {
String stringVal = je.getAsString();
Object ov = this.extractor.extractValue((String)columnKeys.get(index), stringVal, id, serie.getName());
values.add(ov);
} else {
values.add(je.getAsString());
}

++index;
}

Objects.requireNonNull(serie);
serie.addRecord(new InfluxQLQueryResult.Series.Record(serie, values.toArray()));
}
}
```

When it gets the `null` value, the instruction `String stringVal = je.getAsString();` fires the exception.

Am I doing something wrong?