Ensure custom deleter is called by passing non-null pointer in ctor

[anmelle-slamcore]

Custom deleters for std::unique_ptr are not invoked if the owned pointer is null.
Therefore, the function setup to acquire_metadata at the end of the scope will never fire.

Solution is to pass a valid pointer, and let the custom deleter delete the object.

[dorodnic]

Hi @anmelleSlamcore
Thank you for the patch, I was not familiar with this behaviour.

[ev-mp]

Hello @anmelleSlamcore ,
Thank you for the contribution!