Fix Symbol's description default value (#4077)

In the standard if we create a Symbol object without adding a description value
default value is undefined not empty string.

JerryScript-DCO-1.0-Signed-off-by: bence gabor kis kisbg@inf.u-szeged.hu

tests/jerry/es.next/symbol-prototype-description.js

@@ -73,3 +73,6 @@ try {
 } catch (e) {
   assert(e instanceof TypeError);
 }
+
+assert(Symbol("").description !== undefined);
+assert(Symbol().description === undefined);