fix: nil error log when destination refreshed

[BruceMacD]

Summary

This logged a nil error on destination refresh.

{"level":"error","ts":1647282784.513224,"caller":"engine/engine.go:483","msg":"initializing destination: %!w(*errors.errorString=&{error updating existing destination: %!w(<nil>)})"}

Checklist

• Wrote appropriate unit tests
• Considered security implications of the change
• Updated associated docs where necessary
• Updated associated configuration where necessary
• Change is backwards compatible if it needs to be (user can upgrade without manual steps?)
• Nothing sensitive logged
• Commit message conforms to [Conventional Commit][1]