Do not remap undefined heads in best_alignment.
Do not remap undefined heads in best_alignment.

ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe,ufal/udpipe