remove Symbol::empty

the only use of this function is to determine whether a lambda has a non-set
formal, but this use is arguably better served by Symbol::set and using a
non-Symbol instead of an empty symbol in the parser when no such formal is present.

src/libexpr/value-to-xml.cc

@@ -139,7 +139,7 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
             if (v.lambda.fun->hasFormals()) {
                 XMLAttrs attrs;
-                if (!v.lambda.fun->arg.empty()) attrs["name"] = v.lambda.fun->arg;
+                if (v.lambda.fun->arg.set()) attrs["name"] = v.lambda.fun->arg;
                 if (v.lambda.fun->formals->ellipsis) attrs["ellipsis"] = "1";
                 XMLOpenElement _(doc, "attrspat", attrs);
                 for (auto & i : v.lambda.fun->formals->lexicographicOrder())