Dashboard page: scanning stats are wrong when no scan has ever been done

[flavio]

The "days since the last scan" component inside of the dashboard page is wrong when no ScanJob has ever been created.

Steps to reproduce:

• Install SBOMbastic, do not define any Registry
• Visit the Dashboard page of the UI extension

Version 0.20.0

[xingzhang-suse]

After fixed, we will show a statement if there is no scan started yet