On the master branch, currently PDF generation is not possible. FOP fails to recognize the new version of the Google Noto Fonts introduced with PR #5807.
It seems the font file format changed (Font file signature says something about "Noto SansRegular2.000") enough that FOP cannot work with these fonts.
Therefore, it's best to rollback that commit and use the original version of the fonts again.
Created Pull Request #5907