allow a NULL value in species_set.genome_db_id for the purpose of auto-incrementing the species_set_id safely
allow a NULL value in species_set.genome_db_id for the purpose of auto-incrementing the species_set_id safely

Ensembl/ensembl-compara,ckongEbi/ensembl-compara,kumarsaurabh20/ensembl-compara,etapanari/ensembl-compara,dbolser-ebi/ensembl-compara,danstaines/ensembl-compara,Ensembl/ensembl-compara,kumarsaurabh20/ensembl-compara,dbolser-ebi/ensembl-compara,dbolser-ebi/ensembl-compara,ckongEbi/ensembl-compara,danstaines/ensembl-compara,etapanari/ensembl-compara,Ensembl/ensembl-compara,kumarsaurabh20/ensembl-compara,etapanari/ensembl-compara,danstaines/ensembl-compara,kumarsaurabh20/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,danstaines/ensembl-compara,Ensembl/ensembl-compara,Ensembl/ensembl-compara,ckongEbi/ensembl-compara,etapanari/ensembl-compara,danstaines/ensembl-compara,Ensembl/ensembl-compara