Fix crash on unconnectable MySQL server (resolves #33582)

[danslimmon]

What does this PR do?

Fixes a stacktrace-inducing crash when the mysql module was used against a server to which it couldn't connect. Several of the functions in the mysql module didn't check whether the connection was successful before attempting to grab a cursor. Now they do.

What issues does this PR fix or reference?

#33582

Previous Behavior

$ sudo ./salt/scripts/salt saltdev mysql.get_slave_status
saltdev:
    The minion function caused an exception: Traceback (most recent call last):
      File "/home/ubuntu/salt/salt/minion.py", line 1078, in _thread_return
        return_data = func(*args, **kwargs)
      File "/home/ubuntu/salt/salt/modules/mysql.py", line 1959, in get_slave_status
        rtnv = __do_query_into_hash(conn, "SHOW SLAVE STATUS")
      File "/home/ubuntu/salt/salt/modules/mysql.py", line 1837, in __do_query_into_hash
        cursor = conn.cursor()
    AttributeError: 'NoneType' object has no attribute 'cursor'

New Behavior

$ sudo ./salt/scripts/salt saltdev mysql.get_slave_status
saltdev:

It now returns an empty array, just like the other module functions do. The states, for their part, detect the error from __context__ and fail if there was a problem with the connection.

Tests written?

Yup: unit.modules.mysql_test.MySQLTestCase.test_get_slave_status_bad_server

[danslimmon]

Hard to tell whether the broken tests here are my fault. Some are apt mutex errors, others not. What do you think?

[cachedout]

This looks good to me. Thanks, @danslimmon