Fix mapped_hmset with empty hash

If you try to use `mapped_hmset` with an empty hash, like `r.mapped_hmset("foo", {})`, the following exception is raised:

    Redis::CommandError: ERR wrong number of arguments for 'hmset' command
        /Users/juarez.bochi/projects/redis-rb/lib/redis/client.rb:97:in `call'
        /Users/juarez.bochi/projects/redis-rb/lib/redis.rb:1795:in `block in hmset'
        /Users/juarez.bochi/projects/redis-rb/lib/redis.rb:37:in `block in synchronize'