Support attrs is None.

--HG--
branch : distribute
extra : rebase_source : 7ab3d57b13cb2ec2d7d1b9aafc99f4ec0ea49b5a

setuptools/dist.py

@@ -210,7 +210,7 @@ class Distribution(_Distribution):
         self.require_features = []
         self.features = {}
         self.dist_files = []
-        self.src_root = attrs.pop("src_root", None)
+        self.src_root = attrs and attrs.pop("src_root", None)
         self.patch_missing_pkg_info(attrs)
         # Make sure we have any eggs needed to interpret 'attrs'
         if attrs is not None: