Prepared_stmt.fetch() should explicitly either return None or an empty list

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* **asyncpg version**: 0.18.3
* **PostgreSQL version**: 11
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: Install is local
* **Python version**: 3.7.3
* **Platform**: Windows 10
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**: N/A
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: Have not attempted at this time

This is a great help, thank you for this module!

Since the doc says fetch() returns a list of Records, I initially tried to use it to check if something exists in my database by checking it for a len() of 0. However, this produced unexpected results (but no clear error), so I went to check the documentation again.

While fetchval and fetchrow both explicitly return None if __bind_execute does not return any data, it's not made clear what will be returned if the data is not found in the general fetch() method. Protocol.bind_execute doesn't clearly state this either.

I think it would make most sense to return an empty list, but if it returns None instead that also makes sense with the result of that function 'family.' Either way it would be great to make this explicit in the method itself!

Inizia da Prepared_stmt.fetch() e confronta il suo comportamento documentato con fetchval() e fetchrow(), che l’issue identifica come controparti esplicite. Conferma il risultato quando non ci sono righe e rendi esplicito il contratto di restituzione di fetch(), incluso se si tratta di None o di una lista vuota; aggiungi o aggiorna la copertura di questo comportamento se i test esistenti del progetto lo evidenziano.

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