Add hlint warning for wrong use of 'length'

While removing split instances check from hcheck, I saw a wrong use of
'length': this should only be used when one needs the actual length of
the list, and not when one wants to check whether the list is empty or
not (in that case, 'null' should be used).

To prevent this in the future, let's add a hlint hint for it, and fix
the warning it generates in hbal.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>