Don't panic if a queue exists, but is empty when repairing metadata.

service/src/main/resources/lua/repair_queue_metadata.lua

@@ -1,13 +1,10 @@
-local queueKey           = KEYS[1]
-local queueMetadataKey   = KEYS[2]
+local queueKey         = KEYS[1]
+local queueMetadataKey = KEYS[2]
 
-local firstMessageWithScore = redis.call("ZRANGE", queueKey, 0, 0, "WITHSCORES")
-local lastMessageWithScore  = redis.call("ZRANGE", queueKey, -1, -1, "WITHSCORES")
-
-if firstMessageWithScore ~= nil and lastMessageWithScore ~= nil then
-    local firstMessageId = tonumber(firstMessageWithScore[2])
-    local lastMessageId  = tonumber(lastMessageWithScore[2])
+local firstMessageId = tonumber(redis.call("ZRANGE", queueKey, 0, 0, "WITHSCORES")[2])
+local lastMessageId  = tonumber(redis.call("ZRANGE", queueKey, -1, -1, "WITHSCORES")[2])
 
+if firstMessageId and lastMessageId then
     for messageId = firstMessageId,lastMessageId do
         if redis.call("ZRANGEBYSCORE", queueKey, messageId, messageId) then
             -- This message actually exists, and its GUID may be pointing to the wrong ID

service/src/test/java/org/whispersystems/textsecuregcm/storage/MessagesCacheTest.java

@@ -120,6 +120,12 @@ public class MessagesCacheTest extends AbstractRedisClusterTest {
         assertTrue(messagesCache.getMessagesToPersist(DESTINATION_UUID, DESTINATION_DEVICE_ID, 100).isEmpty());
     }
 
+    @Test
+    public void testRepairEmptyQueueMetadata() {
+        // We're happy as long as this doesn't throw an exception
+        messagesCache.repairMetadata(DESTINATION_UUID, DESTINATION_DEVICE_ID);
+    }
+
     @Test
     @Parameters({"true", "false"})
     public void testRemoveById(final boolean sealedSender) {