fix: Only close output midi port if an underlying client exists

[hlxid]

If the underlying RtMidiOut client could not be successfully created in the constructor handle will be set to a nullptr.

Before this patch the closePort method would be called on a null value resulting in a segmentation fault that crashes the whole node.js process in which the library is used.

The input class already has this check implemented, it is only missing in the output class.

[Julusian]

Thanks, I don't know why I didn't spot/think of this when doing 2f4d70a65cc2ec3f424fc8ea24c060826b15c220

[Julusian]

This is published in v3.0.1