chore: fix the bintray-debian publishing script (#930)
chore: fix the bintray-debian publishing script (#930)

This change prevents the script erroring with `BINTRAY_USER: unbound variable` when `$BINTRAY_USER` isn't defined, and instead displays the "Please define the following ..." message as expected.
resin-io/etcher,resin-io/herostratus,resin-io/resin-etcher,resin-io/etcher,resin-io/etcher,resin-io/resin-etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher