test: fix confusing tuple value

`null` is a common variable in Lua. Since the variable is not declared
and strict mode is off, it is essentially `nil`.

test/suites/test_schema.py

@@ -79,7 +79,7 @@ def setUpClass(cls):
                 if_not_exists = true,
             })
 
-            box.space.tester:insert({1, null})
+            box.space.tester:insert({1, nil})
 
             return true
         """)