remove NOT NULL constraint on unmapped_object.external_db_id
remove NOT NULL constraint on unmapped_object.external_db_id

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