Fix uninitialized field in Attr constructor

The default constructor for Attr was not initializing the value pointer,
which could lead to undefined behavior when the uninitialized pointer is
accessed. This was caught by clang-tidy's UninitializedObject check.

This fixes the warning:
  1 uninitialized field at the end of the constructor call
  [clang-analyzer-optin.cplusplus.UninitializedObject]

src/libexpr/include/nix/expr/attr-set.hh

@@ -23,7 +23,7 @@ struct Attr
        way we keep Attr size at two words with no wasted space. */
     Symbol name;
     PosIdx pos;
-    Value * value;
+    Value * value = nullptr;
     Attr(Symbol name, Value * value, PosIdx pos = noPos)
         : name(name), pos(pos), value(value) { };
     Attr() { };