Routerset config parsing: represent empty sets as NULL.

routerset_t has two representations of an empty routerset: NULL, and
a set containing no elements.  But some of our config code assumes
that empty routersets are represented as NULL.  So let's give it
what it assumes.

Fixes bug 31495. Bugfix on e16b90b88a76; but not in any released
Tor.

src/feature/nodelist/routerset.c

@@ -479,6 +479,10 @@ routerset_kv_parse(void *target, const config_line_t *line, char **errmsg,
     *errmsg = tor_strdup("Invalid router list.");
     return -1;
   } else {
+    if (routerset_is_empty(rs)) {
+      /* Represent empty sets as NULL. */
+      routerset_free(rs);
+    }
     *p = rs;
     return 0;
   }
@@ -507,8 +511,10 @@ routerset_copy(void *dest, const void *src, const void *params)
   routerset_t **output = (routerset_t**)dest;
   const routerset_t *input = *(routerset_t**)src;
   routerset_free(*output); // sets *output to NULL
-  *output = routerset_new();
-  routerset_union(*output, input);
+  if (! routerset_is_empty(input)) {
+    *output = routerset_new();
+    routerset_union(*output, input);
+  }
   return 0;
 }