hashmap: allow NULL key in ordered_hashmap_next()

There is no reason to require key to be non-NULL.
Change test_ordered_hashmap_next() to use trivial_hash_ops in order to
test NULL key too.

src/basic/hashmap.c

@@ -1798,8 +1798,6 @@ void *ordered_hashmap_next(OrderedHashmap *h, const void *key) {
         struct ordered_hashmap_entry *e;
         unsigned hash, idx;
 
-        assert(key);
-
         if (!h)
                 return NULL;