Fix taskweek.php undefined variables

[dragonflyfree]

Checklist

• I'm using the correct PHP Version (7.4 for current, 7.2 for legacy).
• I've added comments to any new methods I've created or where else relevant.
• I've replaced magic method usage on DbService classes with the getInstance() static method.
• I've written any documentation for new features or where else relevant in the docs repo.

Fixes #202 by checking whether members or group exist and, if not, send them to the template as an empty array instead of null

Changelog

refs:
issues: #202

Other Information

Docs pull request: