Monitoring→Problems: Incorrect filter results with empty multiselect fields

Monitoring→Problems: Incorrect filter results when clear multiselect fields after previous filtering.

Steps to reproduce:

1. Create some problems
2. Navigate to Monitoring→Problems
3. Fill multiselect field (Host groups, Hosts, Triggers)
4. Press Apply
5. Filter results are correct
6. Clear multiselect field
7. Press Apply
8. Observe that filtering results are the same as in Step 5

Result:
Clearing multiselect fields does not affect filter results.

Expected:
Filtering result shows all corresponding data for all entities.