Support balancing through trino-lb

[Alexxz]

I'd like to suggest a small and trivial fix for trino-python-client.
To make it working through https://github.com/stackabletech/trino-lb I had to make a small change.
Instead following line

    if "error" in response:

write

    if response.get("error") is not None:

Without this change there is TypeError: 'NoneType' object is not subscriptable error from the line below it

The key difference is having a key in JSON with null (None) value.
I've tried to figure out what is the correct behavior for for the protocol (whether it is the issue in python driver of in trino-lb project).

The documentation says following about the error attribute: "If query failed, the error attribute contains a QueryError object. That object contains a message, an errorCode and other information about the error. See the io.trino.client.QueryError class in module trino-client in the client directory for more details."

In the source code of Trino the object QueryResult is serialized to JSON from structure using Jackson library. So the null value of field is omitted in the serialization. It does not specify any attributes about this field.

I think that generally speaking statically typed languages such as Go, Java, Rust by default don't make any difference on cases if key is missing or its value is null.

Giving all these thoughts above I feel that suggested change in the trino-python-client worth to be done