Change workspaces agent config dns_zone default to empty string

What does this MR do and why?

Change workspaces agent config dns_zone default to empty string. We want to do this so that we can allow this field to become optional in Update workspace agent config dns zone validations (!205043 - merged) .

References

• Inject agentw in workspace (#569402 - closed)
• !205043 (comment 2752634419)

DB migrations

Up

$ scripts/database/migrate.rb        
$ bin/rails db:migrate:up:ci db:migrate:up:main VERSION=20250917093907
ci: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74938
ci: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: migrating ========
ci: -- change_column_default(:workspaces_agent_configs, :dns_zone, {:from=>nil, :to=>""})
ci:    -> 0.0288s
ci: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: migrated (0.0372s) 

ci: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74938
main: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74970
main: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: migrating ========
main: -- change_column_default(:workspaces_agent_configs, :dns_zone, {:from=>nil, :to=>""})
main:    -> 0.0037s
main: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: migrated (0.0071s) 

main: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74970

Down

$ scripts/database/migrate.rb -t down                  
$ bin/rails db:migrate:down:ci db:migrate:down:main VERSION=20250917093907
ci: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74681
ci: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: reverting ========
ci: -- change_column_default(:workspaces_agent_configs, :dns_zone, {:from=>"", :to=>nil})
ci:    -> 0.0211s
ci: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: reverted (0.0296s) 

ci: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74681
main: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74716
main: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: reverting ========
main: -- change_column_default(:workspaces_agent_configs, :dns_zone, {:from=>"", :to=>nil})
main:    -> 0.0039s
main: == 20250917093907 ChangeWorkspacesAgentConfigDnsZoneDefault: reverted (0.0071s) 

main: == [advisory_lock_connection] object_id: 154940, pg_backend_pid: 74716

Screenshots or screen recordings

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How to set up and validate locally

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