Make the root symbol more telling in the DB

I spent a few minutes trying to understand why I had a field with an
empty symbol, until I realise that it’s because that was the symbol for
the root element.
Now that shouldn’t happen anymore

src/libexpr/tree-cache.cc

@@ -209,7 +209,7 @@ Cache::Cache(const Hash & useCache,
         SymbolTable & symbols)
     : db(std::make_shared<AttrDb>(useCache))
     , symbols(symbols)
-    , rootSymbol(symbols.create(""))
+    , rootSymbol(symbols.create("%root%"))
 {
 }