fix: account for None in error field of http response

[adrianAzoitei]

Description

When the error field in the REST response is null (None for the python client), assume a successful execution.

From the docs:

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.

queryError | QueryError | Non-null only if the query resulted in an error.

However, sometimes (some versions of Trino) the server returns an HTTP response with the error field present, yet with a None value. Then, an exception is thrown in the client:

Traceback (most recent call last):
  File "/OBFUSCATED/main.py", line 29, in <module>
    main()
  File "/OBFUSCATED/main.py", line 25, in main
    asyncio.run(trino_scraper.start())
  File "/root/.pyenv/versions/3.11.10/lib/python3.11/asyncio/runners.py", line 190, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "/root/.pyenv/versions/3.11.10/lib/python3.11/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.pyenv/versions/3.11.10/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/OBFUSCATED/scrapers.py", line 165, in start
    cur.execute(
  File "/OBFUSCATED/.venv/lib/python3.11/site-packages/trino/dbapi.py", line 585, in execute
    self._iterator = iter(self._query.execute())
                          ^^^^^^^^^^^^^^^^^^^^^
  File "/OBFUSCATED/.venv/lib/python3.11/site-packages/trino/client.py", line 814, in execute
    status = self._request.process(response)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/OBFUSCATED/.venv/lib/python3.11/site-packages/trino/client.py", line 619, in process
    raise self._process_error(response["error"], response.get("id"))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/OBFUSCATED/.venv/lib/python3.11/site-packages/trino/client.py", line 584, in _process_error
    error_type = error["errorType"]
                 ~~~~~^^^^^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable

Non-technical explanation

Sometimes (some versions of Trino) the server returns an HTTP response with the error field present, yet with a None value.

Release notes

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Fixes #511

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