[9.20] fix: usr: AMTRELAY type 0 presentation format handling was wrong

RFC 8777 specifies a placeholder value of "." for the gateway field when the
gateway type is 0 (no gateway).  This was not being checked for nor emitted
when displaying the record. This has been corrected.

Instances of this record will need the placeholder period added to them when upgrading.

Closes #5639

Backport of MR !11240

Merge branch 'backport-5639-fix-atmrelay-type-0-support-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11255