Add Obtain Null Broadcast Events function to DQMH module

[Olivier-JOURDAN]

Original idea from @FabiolaDelaCueva 

Sometimes you could need to unregister broadcast events. To do this you want to use the Broadcast Events--cluster.ctl, but it's set to private scope (it's totally fine).

Having a public wrapper to this control just giving access to the type and not the current event like you can get with Obtain Broadcast Events For Registration function would be great.

Considering providing a fixer to add this wrapper to all other existing modules.

[joerg.hampel]

When talking/teaching to new LV/DQMH users and showing how to prepare the broadcast registration by using the "Obtain Broadcast References for Registration" to define the event types before the corresponding module has even been started, it always is confusing to them that we still have to register (i.e. wire the actual live broadcast references) after the module has actually been started. 

While I would prefer to make the typedef public so I can just wire a constant to the event registration node, a wrapper VI that clearly documents that an empty constant is being returned would already go a long way.

[Olivier-JOURDAN]

Issue #749 created