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/basic/list.h

@@ -170,9 +170,6 @@
              i != (p);                                                  \
              i = i->name##_next ? i->name##_next : (head))
 
-#define LIST_IS_EMPTY(head)                                             \
-        (!(head))
-
 /* Join two lists tail to head: a->b, c->d to a->b->c->d and de-initialise second list */
 #define LIST_JOIN(name,a,b)                                             \
         do {                                                            \