ci: doc-publish: Publish API coverage docs only when available

The API coverage documentation support was (relatively) recently introduced
in bad87f16 and is not available in older
release branches.

Since doc-publish workflows always run in the default (main) branch
context, it is necessary to make the API coverage documentation handling
optional for the workflow runs that are triggered from older release
branches.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>