Definition import: ignore virtual host default queue type when it is set to undefined

[michaelklishin]

To be more defensive.

See #10469 for the background.

[michaelklishin]

The value of undefined is not only accepted, it is the default value. From rabbit_definitions:

add_vhost(VHost, ActingUser) ->
    Name             = maps:get(name, VHost, undefined),
    IsTracingEnabled = maps:get(tracing, VHost, undefined),
    Metadata         = rabbit_data_coercion:atomize_keys(maps:get(metadata, VHost, #{})),
    Description      = maps:get(description, VHost, maps:get(description, Metadata, <<"">>)),
    Tags             = maps:get(tags, VHost, maps:get(tags, Metadata, [])),
    DefaultQueueType = maps:get(default_queue_type, Metadata, undefined),

So, something else must be the culprit in #10469.

[lukebakken]

The problem in #10469 stems from undefined somehow being passed to this function:

https://github.com/rabbitmq/rabbitmq-server/blob/main/deps/rabbit/src/rabbit_queue_type.erl#L241-L251

I'm trying to figure out how that happens, however 🤔

[lukebakken]

Somewhat related issue - #10515