`applied_filters` template tag does not handle `null_option` on form fields

[thatmattlove]

NetBox version

v3.0.2-dev

Python version

3.9

Steps to Reproduce

Prior to 2a293d5, you won't be able to reproduce this because null_option is not properly handled in the UI. After updating...

1. Navigate to IPAM → IP Addresses → Filter
2. Select 'Global' VRF in 'Available VRFs' field
3. Click Search

Expected Behavior

When null_option is present on a selected form field and is not None, the applied filters should display the null_option value.

Observed Behavior

KeyError is raised because vrf_id is not in the cleaned form data.