Recent drug safety updates don't display a published date

[fofr]

specialist-frontend has specific logic for drug safety updates that hides the default published dates:
https://github.com/alphagov/specialist-frontend/blob/923530fd4b4a9e641ef4f94d4d2cb211a9f25821/app/presenters/drug_safety_update_presenter.rb#L4

This is in favour of a first_published_date that used to appear in details.metadata.first_published_at. Looking at older drug safety updates you see something like this:

"metadata": {
  "therapeutic_area": [
    "infectious-disease",
    "urology-nephrology"
  ],
  "first_published_at": "2014-09-25T00:00:00Z",
  "bulk_published": false,
  "document_type": "drug_safety_update"
},

Example: https://www.gov.uk/drug-safety-update/ivabradine-procoralan-in-the-symptomatic-treatment-of-angina-risk-of-cardiac-side-effects

However, in those published more recently (I don't know when the change occurred), the field is missing. This means the page displays without any published date:

Example: https://www.gov.uk/drug-safety-update/valproate-and-developmental-disorders-new-alert-asking-for-patient-review-and-further-consideration-of-risk-minimisation-measures

We are switching rendering from specialist-frontend to government-frontend, we haven't ported the specific date logic which means dates show correctly for new DSUs, but are duplicated on older ones:

Screenshot of this content rendered by government-frontend: https://www.gov.uk/drug-safety-update/nitrofurantoin-now-contraindicated-in-most-patients-with-an-estimated-glomerular-filtration-rate-egfr-of-less-than-45-ml-min-1-73m2

• What is the correct behaviour?
• Should the content be made consistent?
• Is the correct issue date missing from newer updates? (eg date the publication was published)

More context:
alphagov/specialist-frontend#74

Never show a "Published" date for Drug Safety Updates (this is the "issued_date" in the actual document metadata, which appears at the top of the document). Show an "Updated" date if the document isn't the first edition.

alphagov/specialist-frontend@923530f

DSU is unique in that they don't display any date metadata in the footer
if it's the first edition as we would then get a duplicate Published
(which is really first_published_at) and Published, which will change to
Updated when the document is updated. When it's not the first edition we
see Published (which is really first_published_at) and Updated, so
it's not a problem in that state.

cc @tijmenb @whoojemaflip