Harden flipper feature.get access

For ab_feature_enabled makes sure we don't get a runtime error if
flipper implementation for `get` changes

ee/app/models/ee/user.rb

@@ -338,7 +338,7 @@ module EE
 
       # We use a 2nd feature flag for control as enabled and percentage_of_time for chatops
       flipper_feature = ::Feature.get((feature.to_s + '_control').to_sym) # rubocop:disable Gitlab/AvoidFeatureGet
-      percentage ||= flipper_feature.gate_values[:percentage_of_time] || 0 if flipper_feature
+      percentage ||= flipper_feature&.percentage_of_time_value || 0
       return false if percentage <= 0
 
       if filter == UserPreference::FEATURE_FILTER_UNKNOWN

ee/spec/models/user_spec.rb

@@ -982,6 +982,16 @@ describe User do
             expect(experiment_user.ab_feature_enabled?(:discover_security)).to eq(true)
             expect(experiment_user.user_preference.feature_filter_type).to eq(UserPreference::FEATURE_FILTER_EXPERIMENT)
           end
+
+          it 'returns false if flipper returns nil for non-existing feature' do
+            # The following setup ensures that if the Feature interface changes
+            # it does not break any user-facing screens
+            allow(Feature).to receive(:get).with(:discover_security).and_return(nil)
+            allow(Feature).to receive(:enabled?).and_return(true)
+            allow(Feature).to receive(:get).with(:discover_security_control).and_return(nil)
+
+            expect(experiment_user.ab_feature_enabled?(:discover_security)).to eq(false)
+          end
         end
       end
     end