message: Remove null pointer test from first_element_by_path
  All other functions treat null pointer inputs as invalid; now this
  function does as well.
diff: @@ -6132,7 +6132,7 @@ namespace pugi
  {
  xml_node found = *this; // Current search context.
  - if (!_root || !path_ || !path_[0]) return found;
  + if (!_root || !path_[0]) return found;
  if (path_[0] == delimiter)
  {
  