In memory_plan, check if value is not None, instead of just checking …

…value as boolean. (apache#5700)

python/tvm/relay/transform/memory_plan.py

@@ -243,7 +243,7 @@ def mk_let(self, dynamic_regions):
         def _mk_let(bindings, body):
             for var, value in reversed(bindings):
                 assert var
-                assert value
+                assert value is not None
                 assert body
                 body = expr.Let(var, value, body)
                 if var in dynamic_regions: