tools/make-frozen.py: Allow to run with no directory passed in.

In which case it will just emit empty frozen C definitions.

tools/make-frozen.py

@@ -27,10 +27,11 @@ def module_name(f):
 
 modules = []
 
-root = sys.argv[1].rstrip("/")
-root_len = len(root)
+if len(sys.argv) > 1:
+    root = sys.argv[1].rstrip("/")
+    root_len = len(root)
 
-for dirpath, dirnames, filenames in os.walk(root):
+    for dirpath, dirnames, filenames in os.walk(root):
         for f in filenames:
             fullpath = dirpath + "/" + f
             st = os.stat(fullpath)