fix: BaseURL missing when MPD base path is empty (#1380)

The check for `!mpd_dir.empty()` is not needed because MakePathRelative
handles the case where the parent path is empty. As a result of this
check the base url, segment url, or segment template URLs were all
missing in cases where the mpd output was in the current working
directory.

Fixes #1378

packager/mpd/base/mpd_builder.cc

@@ -417,7 +417,6 @@ void MpdBuilder::MakePathsRelativeToMpd(const std::string& mpd_path,
 
   if (!mpd_file_path.empty()) {
     const std::filesystem::path mpd_dir(mpd_file_path.parent_path());
-    if (!mpd_dir.empty()) {
     if (media_info->has_media_file_name()) {
       media_info->set_media_file_url(
           MakePathRelative(media_info->media_file_name(), mpd_dir));
@@ -432,6 +431,5 @@ void MpdBuilder::MakePathsRelativeToMpd(const std::string& mpd_path,
     }
   }
 }
-}
 
 }  // namespace shaka