The following behaviour was previously present and has been fixed:
lix/lix2 » nix eval --expr '{x."" = 2;}' 'x.""'
{ "" = 2; }
lix/lix2 » nix eval --expr '{x."".y = 2;}' 'x."".y'
error: empty attribute name in selection path 'x."".y'
Change-Id: Iad21988f1191c33a3661c72a6b7f01a8b8b3e6eb