Don't expand empty [paths] so later interpolation can do the right thing.

Example:
[paths]
default =
default-push = %(default)s

mercurial/ui.py

@@ -60,7 +60,7 b' class ui(object):'
         if root is None:
             root = os.path.expanduser('~')
         for name, path in self.configitems("paths"):
-            if path.find("://") == -1 and not os.path.isabs(path):
+            if path and path.find("://") == -1 and not os.path.isabs(path):
                 self.cdata.set("paths", name, os.path.join(root, path))
 
     def setconfig(self, section, name, val):