store: added undef check for db_release for is NULL clause
store: added undef check for db_release for is NULL clause

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