Update FunctionDef.infer_call_result to infer None with no Returns

Ref pylint-dev#485. If the function was inferred (unlike many compiler-builtins)
and it contains no Return nodes, then the implicit return value is None.

astroid/scoped_nodes.py

@@ -1744,7 +1744,7 @@ def infer_call_result(self, caller=None, context=None):
 
         first_return = next(returns, None)
         if not first_return:
-            if self.body and isinstance(self.body[-1], node_classes.Assert):
+            if self.body:
                 yield node_classes.Const(None)
                 return
 

tests/unittest_scoped_nodes.py

@@ -466,6 +466,18 @@ def func():
         self.assertIsInstance(func_vals[0], nodes.Const)
         self.assertIsNone(func_vals[0].value)
 
+    def test_no_returns_is_implicitly_none(self):
+        code = """
+            def f():
+                pass
+            value = f()
+            value
+        """
+        node = builder.extract_node(code)
+        inferred = next(node.infer())
+        assert isinstance(inferred, nodes.Const)
+        assert inferred.value is None
+
     def test_func_instance_attr(self):
         """test instance attributes for functions"""
         data = """