Avoid UnboundLocalError in beacons module

[rallytime]

What does this PR do?

If something goes wrong with the event response call, we'll hit an UnboundLocalError stack trace during the if beacons check below. This avoids that stacktrace by instantiating beacons as None before trying the event call.

This imports the correct salt.utils.event lib (not just salt.utils - event.py is it's own file and there isn't an event reference in salt.utils.__init__.py).

What issues does this PR fix or reference?

None that I know of - found during the testing of the

Previous Behavior

When trying to perform a salt-call --local beacons.list function, the stacktrace happens when no beacons are configured:

Traceback (most recent call last):
  File "/usr/bin/salt-call", line 11, in <module>
    salt_call()
  File "/root/SaltStack/salt/salt/scripts.py", line 338, in salt_call
    client.run()
  File "/root/SaltStack/salt/salt/cli/call.py", line 53, in run
    caller.run()
  File "/root/SaltStack/salt/salt/cli/caller.py", line 134, in run
    ret = self.call()
  File "/root/SaltStack/salt/salt/cli/caller.py", line 197, in call
    ret['return'] = func(*args, **kwargs)
  File "/root/SaltStack/salt/salt/modules/beacons.py", line 56, in list_
    if beacons:
UnboundLocalError: local variable 'beacons' referenced before assignment

New Behavior

No more stacktrace:

root@rallytime:~# salt-call --local beacons.list
local:
    ----------
    beacons:
        ----------

Tests written?

No - there are already integration tests covering this feature in general. That's how this bug was found. Those tests started failing during the merge-forward in #34894.