basic/list: drop LIST_IS_EMPTY

This was a trivial wrapper that didn't provide any added value. With more
complicated structures like strvs, hashmaps, sets, and arrays, it is possible
to have an empty container. But in case of a list, the list is empty only when
the head is missing.

Also, we generally want the positive condition, so we replace many
if (!LIST_IS_EMPTY(x)) with just if (x).

src/test/test-list.c

@@ -240,7 +240,7 @@ int main(int argc, const char *argv[]) {
 
         LIST_JOIN(item, head, head2);
         assert_se(head2 == NULL);
-        assert_se(!LIST_IS_EMPTY(head));
+        assert_se(head);
 
         for (i = 0; i < ELEMENTSOF(items); i++)
                 LIST_REMOVE(item, head, &items[i]);