Fix delete operation in Set objects (#3203)

Delete should return false if an element doesn't exist in a set.
ECMA_VALUE_EMPTY wasn't checked in case of a deleted property.

JerryScript-DCO-1.0-Signed-off-by: Daniel Balla dballa@inf.u-szeged.hu

tests/jerry/es2015/set.js

@@ -87,3 +87,7 @@ assert (set.size === 3);
 
 set.clear();
 assert(set.size === 0);
+
+set.add(3);
+assert(set.delete(3));
+assert(!set.delete(3));