Fix rubocop offenses

spec/lib/port_pool_spec.rb

@@ -24,7 +24,7 @@ describe PortPool do
       it 'returns the port' do
         available_ports = described_class.instance_variable_get(:@available_ports)
         described_class.instance_variable_set(:@available_ports, [])
-        expect(described_class.available_port).to be nil
+        expect(described_class.available_port).to be_nil
         described_class.instance_variable_set(:@available_ports, available_ports)
       end
     end

spec/services/exercise_service/push_external_spec.rb

@@ -45,7 +45,7 @@ RSpec.describe ExerciseService::PushExternal do
     end
 
     context 'when response status is success' do
-      it { is_expected.to be nil }
+      it { is_expected.to be_nil }
 
       context 'when response status is 500' do
         let(:status) { 500 }
@@ -58,7 +58,7 @@ RSpec.describe ExerciseService::PushExternal do
     context 'when an error occurs' do
       before { allow(Faraday).to receive(:new).and_raise(StandardError) }
 
-      it { is_expected.not_to be nil }
+      it { is_expected.not_to be_nil }
     end
   end
 end

spec/services/proforma_service/export_task_spec.rb

@@ -16,7 +16,7 @@ describe ProformaService::ExportTask do
       subject(:export_task) { described_class.new }
 
       it 'assigns exercise' do
-        expect(export_task.instance_variable_get(:@exercise)).to be nil
+        expect(export_task.instance_variable_get(:@exercise)).to be_nil
       end
     end
   end