As part of the fix for T51587, I removed the Depth output for non-Multilayer images since it seemed weird that PNGs etc. that don't have a Z pass still get a socket for it. However, I forgot about non-multilayer EXRs, which are a special case that can actually have a Z pass. Therefore, this commit brings back the Depth output for non-multilayer images just like it was in 2.78.