Fix build_path for invariants

[aulemahal]

Reverse logic to allow missing dates on FX data.

Pull Request Checklist:

• [ ] This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• [ ] (If applicable) Documentation has been added / updated (for bug fixes / features).
• [ ] (If applicable) Tests have been added.
• [x] This PR does not seem to break the templates.
• [ ] HISTORY.rst has been updated (with summary of main changes).
  • [ ] Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

In _schema_dates (which generates the "DATES" string), the fast-path for invariant data happend before the optional checks. Invariant data should not have date_start and date_end, so the current code will always fail for xrfreq='fx'.

Does this PR introduce a breaking change?

No.

Other information:

[aulemahal]

Didn't even wait for a successful CI. :cowboy_hat_face: