Re-enabled the None value generating

Author: Xin Dong

test/correctness/gen.py

@@ -183,9 +183,9 @@ def random_id_value():
 def random_value():
     r = global_prng.random()
     while True:
-        # if (r < 0.1) and generator_options.test_nulls:
-            # val = None
-        if (r < 0.2):
+        if (r < 0.1) and generator_options.test_nulls:
+            val = None
+        elif (r < 0.2):
             val = random_float()
         elif (r < 0.4):
             val = random_string(global_prng.randint(1, 8))