Update irbuild-weakref.test

Author: BobTheBuidler

mypyc/test-data/irbuild-weakref.test

@@ -8,7 +8,7 @@ def f(x: object) -> object:
 def f(x):
     x, r0 :: object
 L0:
-    r0 = PyWeakref_NewRef(x, NULL)
+    r0 = PyWeakref_NewRef(x, 0)
     return r0
 
 [case testWeakrefRefCallback]
@@ -34,7 +34,7 @@ def f(x: object) -> object:
 def f(x):
     x, r0 :: object
 L0:
-    r0 = PyWeakref_NewRef(x, NULL)
+    r0 = PyWeakref_NewRef(x, 0)
     return r0
 
 [case testFromWeakrefRefCallback]