libexpr/value: Add constants for the empty set and null

Change-Id: I4b673cbb81b06f1c415a0059cf1238d2d329725e

lix/libexpr/value.cc

@@ -12,8 +12,10 @@ namespace nix
 
 static const Value::List emptyListData{.size = 0};
 Value Value::EMPTY_LIST{Value::list_t{}, &emptyListData};
+Value Value::EMPTY_SET{attrs_t{}, &Bindings::EMPTY};
 
 const Value::Null Value::NULL_ACB = {{Value::Acb::tNull}};
+Value Value::VNULL{null_t{}};
 
 static_assert(alignof(Value::String) >= Value::TAG_ALIGN);
 static_assert(alignof(Bindings) >= Value::TAG_ALIGN);

lix/libexpr/value.hh

@@ -356,6 +356,8 @@ public:
      * Empty list constant.
      */
     static Value EMPTY_LIST;
+    static Value EMPTY_SET;
+    static Value VNULL;
 
     struct String;
     struct Acb;