Fix dashboard card in darkmode

[Elblinator]

Current behaviour:
In darkmode: If a category (future/ended) has no meeting then the corners from the mat-card/the empty card are not in darkmode

screenshot:

Reproduction:

1. navigate to the dashboard
2. turn on darkmode
3. make sure at least one of the different categories has no meeting

Wanted behaviour:
The corners from the card should follow the darkmode theme
-> navigate to meeting > startpage to see one example of the wanted corners

[bastianjoel]

Duplicate to #3512