What if nobody bids above my floor?
Nothing sells and the power goes to your utility at their normal rate, same as today. You never do worse than before.
Then that block does not clear, and your surplus does exactly what it does now: it flows to the grid and your utility credits you at their standard export rate.
This is the whole point of the floor. It is a guarantee, not a gamble. Blocks that do not sell cost you nothing and lose you nothing.
How often does it happen?
Rarely, if your floor is sensible. Across all sellers, about 91% of listed blocks clear. Most unsold blocks are overnight, when supply is low anyway, or from sellers who set very high floors on purpose.
If you notice a lot of unsold blocks, drop your floor a cent or two and watch the feed.