Some maintenance updates missing from `mu_dataset.json`

[adamjanovsky]

Describe the bug

Maintenance updates for which either st_link or report_link does not exist are not properly processed by our pipeline. While they are scraped and recorded in the cert.maintenance_updates variable and displayed on Web, they are not further fetched into CCDatasetMaintenanceUpdates object.

This is due to the fact that the members of CCDatasetMaintenanceUpdates constitute a valid CCCertificate for which both st_link and report_link must be of type str. But this is not the case when either is missing, and so the following method disregards them:

sec-certs/src/sec_certs/sample/cc_maintenance_update.py

Line 77 in 84089f4

def get_updates_from_cc_cert(cls, cert: CCCertificate) -> list[CCMaintenanceUpdate]: