Add default handling in the DefaultCampaignResolver

Steps to reproduce

1. Create a Banner slot
2. Create a default Campaign for the Banner slot
3. Create a normal Campaign for the Banner slot
4. Use the created CampaignManagedComponent and choose the created Banner slot and the created default
5. If the default campaign should be served, no campaign is visible

Expected results

The default campaign should be visible.

Actual results

No Campaign is visible

Workaround

I just created an always active campaign as workaround.

Development notes

The DefaultCampaignResolver should resolve the default campaign if no campaign is active.