Fix variable consistency for optional in fleet_config

[zwinnerman-fleetdm]

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/ or orbit/changes/.
  See Changes files for more information.
• Documented any API changes (docs/Using-Fleet/REST-API.md or docs/Contributing/API-for-contributors.md)
• Documented any permissions changes
• Input data is properly validated, SELECT * is avoided, SQL injection is prevented (using placeholders for values in statements)
• Added support on fleet's osquery simulator cmd/osquery-perf for new osquery data ingestion features.
• Added/updated tests
• Manual QA for all new/changed functionality
  • For Orbit and Fleet Desktop changes:
    • Manual QA must be performed in the three main OSs, macOS, Windows and Linux.
    • Auto-update manual QA, from released version of component to new version (see tools/tuf/test).