Return an empty array instead of throwing ArgumentError if the transa…

…ction is empty in the cluster client

cluster/test/commands_on_transactions_test.rb

@@ -24,9 +24,7 @@ def test_multi
       redis.multi
     end
 
-    assert_raises(ArgumentError) do
-      redis.multi {}
-    end
+    assert_empty(redis.multi {})
 
     assert_equal([1], redis.multi { |r| r.incr('counter') })
   end
@@ -56,6 +54,8 @@ def test_watch
       end
     end
 
+    assert_empty(redis.watch('{key}1', '{key}2') {})
+
     redis.watch('{key}1', '{key}2') do |tx|
       tx.call('SET', '{key}1', '1')
       tx.call('SET', '{key}2', '2')