[recipe_engine] Fix a couple path-related bugs.

recipe_engine/config_types.py

 # Copyright 2013 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import abc
 import os
 import re
 
 from collections import namedtuple
 
@@ skipping 160 matching lines @@
 
   def __eq__(self, other):
     return (self.base == other.base and
             self.pieces == other.pieces and
             self.platform_ext == other.platform_ext)
 
   def __ne__(self, other):
     return not self.base == other
 
   def join(self, *pieces, **kwargs):
+    if not pieces and not kwargs:
+      return self
     kwargs.setdefault('platform_ext', self.platform_ext)
     return Path(self.base, *filter(bool, self.pieces + pieces), **kwargs)
 
   def is_parent_of(self, child):
     """True if |child| is in a subdirectory of this path."""
     # Assumes base paths are not nested.
     # TODO(vadimsh): We should not rely on this assumption.
     if self.base != child.base:
       return False
     # A path is not a parent to itself.
     if len(self.pieces) >= len(child.pieces):
       return False
     return child.pieces[:len(self.pieces)] == self.pieces
 
   def __repr__(self):
     s = "Path(%r" % self.base
     if self.pieces:
       s += ", %s" % ",".join(repr(x) for x in self.pieces)
 
     return s + ")"