General :: Append At A New Line If The Old Line Is Non Empty?

I want to append at a new line if the old line is non empty. so it would be something like

Code:

1
2
3//dont add here

Code:

1
2
3
//add here

but if sometimes there could be extra empty line, then add at the empty line. So the script needs to detect whether or not the last line is empty. i figured out the symbol for a new line pattern is ^ so it would probably be sed "a^stuff here"is this right? but i dont seem to get the right result