Only require external_db_id to be filled in if the type is xref.
Only require external_db_id to be filled in if the type is xref.

willmclaren/ensembl,Ensembl/ensembl,Ensembl/ensembl,danstaines/ensembl,mjg17/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,at7/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,at7/ensembl,mjg17/ensembl,at7/ensembl,muffato/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,danstaines/ensembl,willmclaren/ensembl,Ensembl/ensembl,muffato/ensembl,at7/ensembl,muffato/ensembl,james-monkeyshines/ensembl,mjg17/ensembl