Fix RuboCop 0.40 linter errors (#1722)

These errors are breaking the build, which seems to use RuboCop 0.40 [1]
despite the Gemfile.lock pinning rubocop to 0.38.

New lints that I am updating the code style to reflect:

- Style/EmptyCaseCondition: Do not use empty case condition, instead use
  an if expression.

- Style/MultilineArrayBraceLayout: Closing array brace must be on the
  same line as the last array element when opening brace is on the same
  line as the first array element.

- Style/MultilineHashBraceLayout: Closing hash brace must be on the same
  line as the last hash element when opening brace is on the same line
  as the first hash element.

- Style/MultilineMethodCallBraceLayout: Closing method call brace must
  be on the line after the last argument when opening brace is on a
  separate line from the first argument.

[1] https://github.com/bbatsov/rubocop/releases/tag/v0.40.0

test/action_controller/explicit_serializer_test.rb

@@ -100,7 +100,8 @@ module ActionController
         get :render_array_using_explicit_serializer_and_custom_serializers
 
         expected = [
-          { 'title' => 'New Post',
+          {
+            'title' => 'New Post',
             'body' => 'Body',
             'id' => assigns(:post).id,
             'comments' => [{ 'id' => 1 }, { 'id' => 2 }],

test/action_controller/json_api/errors_test.rb

@@ -7,8 +7,8 @@ module ActionController
         def test_active_model_with_multiple_errors
           get :render_resource_with_errors
 
-          expected_errors_object =
-            { :errors =>
+          expected_errors_object = {
+            :errors =>
               [
                 { :source => { :pointer => '/data/attributes/name' }, :detail => 'cannot be nil' },
                 { :source => { :pointer => '/data/attributes/name' }, :detail => 'must be longer' },

test/action_controller/serialization_test.rb

@@ -294,7 +294,8 @@ module ActionController
           comments: [
             {
               id: 1,
-              body: 'ZOMG A COMMENT' }
+              body: 'ZOMG A COMMENT'
+            }
           ],
           blog: {
             id: 999,
@@ -333,7 +334,8 @@ module ActionController
           comments: [
             {
               id: 1,
-              body: 'ZOMG A COMMENT' }
+              body: 'ZOMG A COMMENT'
+            }
           ],
           blog: {
             id: 999,
@@ -407,7 +409,8 @@ module ActionController
           comments: [
             {
               id: 1,
-              body: 'ZOMG A COMMENT' }
+              body: 'ZOMG A COMMENT'
+            }
           ],
           blog: {
             id: 999,