Resolve some test deprecation warnings

Resolved due to messages like:

> DEPRECATED: Use assert_nil if expecting nil from test/services/request_helper_test.rb:7.
> This will fail in Minitest 6.

test/helpers/application_helper_test.rb

@@ -15,7 +15,7 @@ class ApplicationHelperTest < ActionView::TestCase
 
   test "#t_locale_fallback returns nil for a string with a locale translation" do
     fallback = t_locale_fallback("content_item.schema_name.imported", count: 1, locale: :de)
-    assert_equal nil, fallback
+    assert_nil fallback
   end
 
   test "#t_locale_fallback returns default locale for a string with no locale translation" do

test/models/http_feature_flags_test.rb

@@ -58,7 +58,7 @@ class HttpFeatureFlagsTest < ActiveSupport::TestCase
 
     feature_flag_value = instance.get_feature_flag('USE_MAGIC')
 
-    assert_equal nil, feature_flag_value
+    assert_nil feature_flag_value
   end
 
   test 'get_feature_flag returns feature flag value when feature flag exists' do

test/services/request_helper_test.rb

@@ -4,7 +4,7 @@ class RequestHelperTest < ActiveSupport::TestCase
   test 'get_header returns nil when header does not exist' do
     header_val = RequestHelper.get_header('Govuk-Example-Header', {})
 
-    assert_equal nil, header_val
+    assert_nil header_val
   end
 
   test 'get_header returns header when header exists' do