basic: fix hosed return value in skip_session()

../src/basic/cgroup-util.c: In function ‘skip_session’:
../src/basic/cgroup-util.c:1241:32: error: incompatible types when returning type ‘_Bool’ but ‘const char *’ was expected
 1241 |                         return false;

(cherry picked from commit db8e720984)

src/basic/cgroup-util.c

@@ -1238,7 +1238,7 @@ static const char *skip_session(const char *p) {
                  * here. */
 
                 if (!session_id_valid(buf))
-                        return false;
+                        return NULL;
 
                 p += n;
                 p += strspn(p, "/");