Fix optional args for POST :id/variables

Always use declared_params(include_missing: false)
so that we don't give nils for optional arguments

lib/api/variables.rb

@@ -45,7 +45,7 @@ module API
         optional :protected, type: String, desc: 'Whether the variable is protected'
       end
       post ':id/variables' do
-        variable = user_project.variables.create(declared(params, include_parent_namespaces: false).to_h)
+        variable = user_project.variables.create(declared_params(include_missing: false))
 
         if variable.valid?
           present variable, with: Entities::Variable

spec/requests/api/variables_spec.rb

@@ -82,6 +82,17 @@ describe API::Variables do
         expect(json_response['protected']).to be_truthy
       end
 
+      it 'creates variable with optional attributes' do
+        expect do
+          post api("/projects/#{project.id}/variables", user), key: 'TEST_VARIABLE_2', value: 'VALUE_2'
+        end.to change{project.variables.count}.by(1)
+
+        expect(response).to have_http_status(201)
+        expect(json_response['key']).to eq('TEST_VARIABLE_2')
+        expect(json_response['value']).to eq('VALUE_2')
+        expect(json_response['protected']).to be_falsey
+      end
+
       it 'does not allow to duplicate variable key' do
         expect do
           post api("/projects/#{project.id}/variables", user), key: variable.key, value: 'VALUE_2'