Fix the logging logic in hook_cron

Problem/Motivation

In the hook_cron implementation the following code is called inside foreach loop for every cron_migration entity:

    if (!$import) {
      $logger->notice('No migrations to import.');
    }

See https://git.drupalcode.org/project/migrate_queue_importer/-/blob/2.1.x/m...

So if no migrations were queued, you can see more than a single "No migrations to import." message in logs.

Steps to reproduce

Proposed resolution

Move the mentioned above code block outside the foreach loop.

Remaining tasks

User interface changes

API changes

Data model changes