[9.20] fix: dev: Check validator name when adding EDE text

When a validator is being shut down, the associated name
`val->name` is set to NULL.  This could cause a crash if a worker
thread subsequently added an EDE code with `val->name` in the
extra text.

`validator_addede()` now checks whether the name is NULL before
trying to add it to the extra text.

Closes #5613

Backport of MR !11945

Merge branch 'backport-each-validator-log-after-shutdown-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11977