I think most people understand that "leaving prompts" is not anything like a guarantee of getting something in response to the prompt, and I've never really known anyone to be mad about not getting something for their prompt. (At least, over the years when I ask for fic prompts, no one has ever told me they were mad if I didn't write them something.)

Please don't worry about being somehow obligated to draw something! I, for one, understand that asking for prompts is very often just one way of looking to be inspired when nothing is coming from inside one's own brain. It isn't like I'm winning you in a fannish auction or something; I don't feel like I'm obligating you by leaving a prompt (and I'd feel bad if I thought that leaving you a prompt added to your anxiety!)

I kind of look at it like buying a lotto ticket. I leave a prompt, and I might get something or I might not. But leaving me a prompt costs me nothing but a few seconds, so I like leaving prompts for people.

Lastly, this stuff is supposed to be just for fun. If something isn't fun, don't do it. If halfway through a drawing, you suddenly have reasons to never ever work on it again, that's totally okay.