Fix deprecation calling assert_equal nil
Fix deprecation calling assert_equal nil

Fix the deprecation:
```
DEPRECATED: Use assert_nil if expecting nil from 
test/unit/helpers/locations_options_helper_test.rb:17. This will fail in 
Minitest 6.
```

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall