chase: Allow passing NULL as the empty path to chaseat()

Per coding style, we should accept NULL as the empty path.

src/basic/chase.c

@@ -81,7 +81,6 @@ int chaseat(int dir_fd, const char *path, ChaseFlags flags, char **ret_path, int
         const char *todo;
         int r;
 
-        assert(path);
         assert(!FLAGS_SET(flags, CHASE_PREFIX_ROOT));
         assert(!FLAGS_SET(flags, CHASE_STEP|CHASE_EXTRACT_FILENAME));
         assert(!FLAGS_SET(flags, CHASE_TRAIL_SLASH|CHASE_EXTRACT_FILENAME));

src/test/test-chase.c

@@ -570,6 +570,10 @@ TEST(chaseat) {
         assert_se(streq(result, "."));
         result = mfree(result);
 
+        assert_se(chaseat(tfd, NULL, CHASE_PARENT|CHASE_AT_RESOLVE_IN_ROOT|CHASE_EXTRACT_FILENAME, &result, NULL) >= 0);
+        assert_se(streq(result, "."));
+        result = mfree(result);
+
         /* Test chase_and_openat() */
 
         fd = chase_and_openat(tfd, "o/p/e/n/f/i/l/e", CHASE_MKDIR_0755, O_CREAT|O_EXCL|O_CLOEXEC, NULL);