attr path parser: fix bug in not rejecting empty attr paths, add unparser

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

tests/unit/libexpr-support/tests/libexpr.hh

@@ -8,7 +8,6 @@
 #include "lix/libexpr/nixexpr.hh"
 #include "lix/libexpr/eval.hh"
 #include "lix/libexpr/eval-inline.hh"
-#include "lix/libstore/store-api.hh"
 
 #include "tests/libstore.hh"
 
@@ -20,6 +19,10 @@ namespace nix {
                 initLibExpr();
             }
 
+            EvalState & evalState() {
+                return state;
+            }
+
         protected:
             LibExprTest()
                 : LibStoreTest()