Returns the length of the list stored at `key`. If `key` does not exist, it is
interpreted as an empty list and `0` is returned. An error is returned when the
value stored at `key` is not a list.

@return

@integer-reply: the length of the list at `key`.

@examples

```cli
LPUSH mylist "World"
LPUSH mylist "Hello"
LLEN mylist
```