Allow an absent 'image' in the ubinize configuration.

index 5115db3..ebd5aa0 100644
@@ -561,20 +561,22 @@ int main(int argc, char * const argv[])
            goto out_free;
        }
 
-        fd = open(img, O_RDONLY);
-        if (fd == -1) {
-            sys_errmsg("cannot open \"%s\"", img);
-            goto out_free;
-        }
+        if (img) {
+            fd = open(img, O_RDONLY);
+            if (fd == -1) {
+                sys_errmsg("cannot open \"%s\"", img);
+                goto out_free;
+            }
 
-        verbose(args.verbose, "writing volume %d", vi[i].id);
-        verbose(args.verbose, "image file: %s", img);
+            verbose(args.verbose, "writing volume %d", vi[i].id);
+            verbose(args.verbose, "image file: %s", img);
 
-        err = ubigen_write_volume(&ui, &vi[i], args.ec, st.st_size, fd, args.out_fd);
-        close(fd);
-        if (err) {
-            errmsg("cannot write volume for section \"%s\"", sname);
-            goto out_free;
+            err = ubigen_write_volume(&ui, &vi[i], args.ec, st.st_size, fd, args.out_fd);
+            close(fd);
+            if (err) {
+                errmsg("cannot write volume for section \"%s\"", sname);
+                goto out_free;
+            }
        }
 
        if (args.verbose)