Resolve "gitlab-runner adds "<nil>" lines to syslog logging"

What does this MR do?

This MR should fix a problem where log messages, when sent via syslog, would appear suffixed with a spurious line containing just "<nil>".

Why was this MR needed?

ServiceLogHook.Fire() would send the result of entry.String() (which returns both a string and an error) directly to the associated service.Logger, whose methods accept a varargs interface{}, thus forwarding both values to syslog.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #2453 (closed)