Short-term fix for errors in reading of metadata-only CDL

[sadielbartholomew]

See #197 and #NCAS-CMS/cfdm#128 for details.

Note in response to the CI failing on coverage I've realised that even for this short-term fix I should add some more tests to cover metadata-only CDL, so I need to create and commit those before this can be merged.

[sadielbartholomew]

The new commit adds testing to cover the cases of relevance. Opening and closing to re-trigger the CI tests...

[sadielbartholomew]

Regarding the CI results, the first test job to run to completion failed, cancelling the others, but only due to one sporadic test failure we have seen often before (namely a "ValueError: cannot reshape" on test_Field_indices (test_Field.FieldTest)).

Since it's very likely at least one other test-running job will fail in the same way, I won't re-run the set, but can have reasonable confidence the changeset is sound from that one job outcome. We'll be running the full set of CI jobs again very soon for the release, anyhow.

As for the source dist testing job, it seems there may some issue in the test_release script, else in the workflow in question, but I'll look into that separately. Otherwise linting passes.

Overall, I'm satisfied I can merge this despite some CI failures.