don't return empty tags strings to lua

profiles/lib/tag_cache.lua

@@ -2,26 +2,15 @@ TagCache = {}
 
 function TagCache.get(way,cache,key)
   local v = cache[key]
-  if v then
-    if v == '' then
-      return nil
-    else
-      return v
-    end
-  else
-    v = way:get_value_by_key(key)
   if v == nil then
-      cache[key] = ''
-      return nil
-    else
-      cache[key] = v
-      if v == '' then
+    v = way:get_value_by_key(key)
+    cache[key] = v or false
+    return v
+  elseif v == false then
     return nil
   else
     return v
   end
 end
-  end
-end
 
 return TagCache

src/extractor/scripting_environment_lua.cpp

@@ -34,7 +34,7 @@ namespace
 template <class T>
 auto get_value_by_key(T const &object, const char *key) -> decltype(object.get_value_by_key(key))
 {
-    return object.get_value_by_key(key, "");
+    return object.get_value_by_key(key);
 }
 
 template <class T> double latToDouble(T const &object)