lib: Allow hash_get to sidestep expensive hash key generation in some cases

lib: Allow hash_get to sidestep expensive hash key generation in some cases

There is no need to generate a hash key *if* the hash_alloc_function
is NULL and the hash is empty.

This changed showed a measurable increase in performance for
table hash lookup for tables that were meant to be empty in
bgp( the distance commands ).

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>