Looking for a "null" Sound Input to securely disable the microphone on a MacBook Pro
Check out SoundFlower

https://rogueamoeba.com/freebies/soundflower/

This virtual / software sound device allows sound to be passed from one application to another (so it of course includes virtual output and input devices).

You could achieve your goal by setting the input to soundflower, but making sure nothing it outputting to soundflower... this means soundflower's input would 'hear' nothing!