BUGFIX: Check if site node exists in context before running policy check

[tdausner]

Bugfix issues/4625

• Code follows the PSR-2 coding style
• Tests have been created, run and adjusted as needed
• The PR is created against the lowest maintained branch
• Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• Reviewer - The first section explains the change briefly for change-logs
• Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions

[mhsdesign]

Oops this seems to be a duplicate of this fix #4269

[mhsdesign]

This fix seems to be incomplete so i will close this in favour of #4269 thank you still for your efforts :D

It basically fixes, that the backend will still be usable, but the site won't show up in the return val $sites, although you might be allowed to access it, because the node is in another dimension than the default.