[FIX] FM index: allow empty strings in string sets
[FIX] FM index: allow empty strings in string sets

bestrauc/seqan,xenigmax/seqan,xenigmax/seqan,xenigmax/seqan,xenigmax/seqan,xenigmax/seqan,bestrauc/seqan,bestrauc/seqan,bestrauc/seqan,xenigmax/seqan,bestrauc/seqan,xenigmax/seqan,bestrauc/seqan,bestrauc/seqan,xenigmax/seqan,bestrauc/seqan