Question

What would happen if the switch was cleared of all MAC addresses?

Answer 1

If an Alternate MAC Address was never entered, clearing it does nothing except restart the console because the field is already blank. In short, clearing the alternate MAC address essentially became a workaround meant to eliminate the guess work that went into manually powering the console down.
So when you disconnect a device, the entry will be removed after some times.
Additionally some switches will immediately remove all entries listing a port if this port goes down The entry in the switch mac table has a timestamp and all records have a lifetime. (300 seconds is a common value but it can be another value, and it may be configurable, depending on the switch vendor / model / software version).
Things become more complex when you have many switches interconnected.