1200 is VFR, which generally means “I’m flying visually and don’t need ATC help”. If you’re squawked 1200 you show up as VFR on their screen, but you still show up. That doesn’t mean null. 7500, 7600, and 7700 are used for various emergency purposes, with 7700 being the most common, and almost always accompanying a mayday or panpan. Those are transponder codes, not radio channels. It’s a code returned by your equipment when the transponder is painted with interrogative radar.

0000 is closer to “null”, but still isn’t quite. 1000 also has some “null” like properties when it comes to ADS-B. Note that what I’m saying is North American centric and not necessarily ICAO nor other areas, which can differ somewhat.