IIRC, The entire contract is not invalid, just the specific clause that says that.

The specific clause is:

    Except as provided in this chapter, every contract by which anyone is restrained from engaging in a lawful profession, trade, or business of any kind is to that extent void.

Most of the exceptions are regarding people who own and then sell the business, and a few others.

Which is to say that your employer would remain powerless, that's how.