...
for(let g = 1; g < 6; g++) {
	for(let p = 1; p < 6; p++) {
		if(document.getElementById("sub_class_" + g + "_" + p).style.backgroundImage == "") {
			if(found_pos == 0) {
				empty_row = "_" + g + "_" + p;
				found_pos = 1;
			}
		}
	}
}
if(found_pos == 1) {
	if(document.getElementById("sub_class" + empty_row)) {
...