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

Nobody has claimed this yet.

* **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!

Start at Prepared_stmt.fetch() and compare its documented behavior with fetchval() and fetchrow(), which the issue identifies as explicit counterparts. Confirm the no-row result and make the fetch() return contract explicit, including whether it is None or an empty list; add or update coverage for that behavior if the existing project tests expose it.

Written by the indexing model from the issue text.