py/objgenerator: Fix handling of None passed as 2nd arg to throw().

Fixes issue #4527.

tests/basics/generator_throw.py

@@ -28,8 +28,8 @@ except StopIteration:
 def gen():
     try:
         yield 123
-    except GeneratorExit:
-        print('GeneratorExit')
+    except GeneratorExit as e:
+        print('GeneratorExit', repr(e.args))
     yield 456
         
 # thrown a class
@@ -41,3 +41,13 @@ print(g.throw(GeneratorExit))
 g = gen()
 print(next(g))
 print(g.throw(GeneratorExit()))
+
+# thrown an instance with None as second arg
+g = gen()
+print(next(g))
+print(g.throw(GeneratorExit(), None))
+
+# thrown a class and instance
+g = gen()
+print(next(g))
+print(g.throw(GeneratorExit, GeneratorExit(123)))