Fix crash for operator policy HTTP API (backport #14717)

[mergify]

This commit fixes the following crash for a GET or a PUT on HTTP API path /api/operator-policies/vhost/name if the supplied vhost doesn't exists:

crasher:
  initial call: cowboy_stream_h:request_process/3
  pid: <0.688.0>
  registered_name: []
  exception error: no function clause matching
                   rabbit_db_rtparams:get({not_found,<<"operator_policy">>,
                                           <<"my_policy">>}) (rabbit_db_rtparams.erl:145)
    in function  rabbit_runtime_parameters:lookup0/2 (rabbit_runtime_parameters.erl:363)
    in call from rabbit_runtime_parameters:lookup/3 (rabbit_runtime_parameters.erl:336)
    in call from rabbit_policy:lookup_op/2 (rabbit_policy.erl:326)
    in call from rabbit_mgmt_wm_operator_policy:resource_exists/2 (rabbit_mgmt_wm_operator_policy.erl:38)
    in call from cowboy_rest:call/3 (src/cowboy_rest.erl:1577)
    in call from cowboy_rest:expect/6 (src/cowboy_rest.erl:1560)
    in call from cowboy_rest:upgrade/4 (src/cowboy_rest.erl:281)

Also, instead of returning a 500 response code, RabbitMQ will now return a 404 response code.

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This is an automatic backport of pull request #14717 done by [Mergify](https://mergify.com).