[9.20] fix: usr: Fix crash on badly configured secondary signer

A badly configured secondary signer that was missing the 'file' entry caused the server to crash, rather than to reject the configuration. This has been fixed.

Closes #5993

Backport of MR !12045

Merge branch 'backport-5993-fix-bump-in-the-wire-crash-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12112