Allow for an empty / null inputs to Tags by removing that frame

[czipperz]

No description provided.

[coveralls]

Coverage decreased (-0.5%) to 68.068% when pulling 90f32ad on czipperz:master into c3917e8 on mpatric:master.

[tmzkt]

This change makes setTrack inconsistent with the rest of the setters. Also, the pull request is missing tests for your proposed change.
The clearFrameSet method would provide you the ability to remove the track number. What is the motivation for keeping the frame with a blank string in it?

[czipperz]

Honestly when writing my app using this, I found it frustrating that my use case (deleting these specific frames) wasn't supported. Now this allows null or "" (empty string). They remove the frame.

[czipperz]

I don't understand how it thinks I am decreasing coverage. It seems it is counting my tests (which are in a separate file) as source code.