rate_limiter: Fix too early return if no rules are passed in.

In the redis implementation, if rules was an empty list,
this would return too early - before checking if the key isn't
manually blocked.

zerver/lib/rate_limiter.py

@@ -222,9 +222,6 @@ class RedisRateLimiterBackend(RateLimiterBackend):
         "Returns a tuple of (rate_limited, time_till_free)"
         list_key, set_key, blocking_key = cls.get_keys(entity_key)
 
-        if len(rules) == 0:
-            return False, 0.0
-
         # Go through the rules from shortest to longest,
         # seeing if this user has violated any of them. First
         # get the timestamps for each nth items
@@ -250,6 +247,9 @@ class RedisRateLimiterBackend(RateLimiterBackend):
                 blocking_ttl = int(blocking_ttl_b)
             return True, blocking_ttl
 
+        if len(rules) == 0:
+            return False, 0.0
+
         now = time.time()
         for timestamp, (range_seconds, num_requests) in zip(rule_timestamps, rules):
             # Check if the nth timestamp is newer than the associated rule. If so,