Optionally ignore null-valued derivation attributes

This allows adding attributes like

  attr = if stdenv.system == "bla" then something else null;

without changing the resulting derivation on non-<bla> platforms.

We once considered adding a special "ignore" value for this purpose,
but using null seems more elegant.

src/libexpr/eval.hh

@@ -93,7 +93,7 @@ public:
     SymbolTable symbols;
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName,
-        sSystem, sOverrides, sOutputName;
+        sSystem, sOverrides, sOutputName, sIgnoreNulls;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */