Add tasks for all the new languages

Task/Null-object/Sidef/null-object-1.sidef

@@ -0,0 +1,3 @@
+var undefined;         # initialized with an implicit nil
+say undefined==nil;    # true
+say defined(nil)       # false

Task/Null-object/Sidef/null-object-2.sidef

@@ -0,0 +1,3 @@
+var null_obj = null;        # initialize with a null value
+say null_obj.is_a(null);    # true
+say defined(null_obj);      # true