remove check for empty canvases

I'd ported this code over from Brandon's change, and in hindsight don't think that it's a good idea, as there are ways for a user to hit this.  For example, on level 1_10 of the artist, the answer is a blank canvas (level is essentially just a sandbox).  If user were to remove all blocks, or just have an empty loop or something they would run into this error.