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

Fixes issue #4527.

tests/basics/gen_yield_from_throw.py

@@ -1,8 +1,8 @@
 def gen():
     try:
         yield 1
-    except ValueError:
-        print("got ValueError from upstream!")
+    except ValueError as e:
+        print("got ValueError from upstream!", repr(e.args))
     yield "str1"
     raise TypeError
 
@@ -16,3 +16,21 @@ try:
     print(next(g))
 except TypeError:
     print("got TypeError from downstream!")
+
+# passing None as second argument to throw
+g = gen2()
+print(next(g))
+print(g.throw(ValueError, None))
+try:
+    print(next(g))
+except TypeError:
+    print("got TypeError from downstream!")
+
+# passing an exception instance as second argument to throw
+g = gen2()
+print(next(g))
+print(g.throw(ValueError, ValueError(123)))
+try:
+    print(next(g))
+except TypeError:
+    print("got TypeError from downstream!")