Returns the values associated with the specified fields in the hash stored at
key.
For every field that does not exist in the hash, a nil value is returned.
Because non-existing keys are treated as empty hashes, running HMGET against
a non-existing key will return a list of nil values.
127.0.0.1:6379> HSET myhash field1 "Hello"
(integer) 1
127.0.0.1:6379> HSET myhash field2 "World"
(integer) 1
127.0.0.1:6379> HMGET myhash field1 field2 nofield
1) "Hello"
2) "World"
3) (nil)