IMHO, I would put some 3/8" rebar in it for reinforcement. If it did crack/move the foundation would stay in one piece. Do a search on floating foundations to get some ideas.

Do you really need a concrete floor? For a little shed like that you could make a wood floor and just let it float on a bed of gravel. If you don't want it to become airborne in a tornado you could do what I did. I dug four 4' deep holes with a post hole digger and set pressure treated 6x6's in them. Then I lag bolted the 6x6's to the floor joists of my shed. Hasn't gone anywhere in 15 years.

I have a 12 x20 that is on 18" of gravel and sleeper 4"x4"'s every two feet. It has been good for 20 years, though a groundhog dug a burrow under shed in the gravel last year.

The secret to sleepers on gravel in a hevay frost climate is to have the gravel well drained. I sloped the excavation to one outside corner and used some drain pipe to make sure that all would drain well.

I built a 8x12 shed in MA and I put in on 4x4x16 and 2x4x16 blocks. If your not putting in anything heavy, no need for a concrete floor. Use pressure treated 3/4 inch tongue and groove plywood over 2x6 for the floor on top of 4x4x16 concrete blocks. Put the blocks on 3/4 crushed gravel. I removed the grass and dug down about 6 inches. I filled the holes with 3/4 gravel, leveled the blocks and made sure they were all the same height and in line with each other. If you are a little off, you can use regular asphalt shingles as shims. If you use multiple blocks and shims, make sure you put a layer construction adhesive between each block/shim. I spread the rest of the gravel under the shed to aide drainage.