How can one filter a column by condition NOT NULL

**Describe the bug**
It's not clear from the documentation how one can filter the subgraph results by a column which should not be NULL.

**To Reproduce**
My (incorrect) query:

```
latest_markets = subgraph.Query.fixedProductMarketMakers(
orderBy=subgraph.FixedProductMarketMaker.creationTimestamp,
orderDirection="desc",
first=2,
where=[str(subgraph.FixedProductMarketMaker.resolutionTimestamp) != "null"], # problem here
)
```

**Expected behavior**
I would expect an example to be found in the documentation.

Look at the subgrounds documentation and code for filtering examples, particularly in the query builder. Check how GraphQL filters are constructed in the library. The goal is to add a clear example of a NOT NULL filter to the docs, showing the correct syntax.

Scritto dal modello di indicizzazione a partire dal testo della issue.