small bugfix prevents model from adding empty gene
small bugfix prevents model from adding empty gene

JuBra/cobrapy,JuBra/cobrapy,zakandrewking/cobrapy,jerkos/cobrapy,jerkos/cobrapy,aebrahim/cobrapy,zakandrewking/cobrapy,aebrahim/cobrapy,jeicher/cobrapy,jeicher/cobrapy