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/serializable_resource_test.rb

@@ -48,12 +48,12 @@ module ActiveModelSerializers
           resource, {
             serializer: ActiveModel::Serializer::ErrorSerializer,
             adapter: :json_api
-          })
-        expected_response_document =
-          { :errors =>
-            [
-              { :source => { :pointer => '/data/attributes/name' }, :detail => 'must be awesome' }
-            ]
+          }
+        )
+        expected_response_document = {
+          :errors => [
+            { :source => { :pointer => '/data/attributes/name' }, :detail => 'must be awesome' }
+          ]
         }
         assert_equal serializable_resource.as_json(options), expected_response_document
       end
@@ -69,12 +69,12 @@ module ActiveModelSerializers
             serializer: ActiveModel::Serializer::ErrorsSerializer,
             each_serializer: ActiveModel::Serializer::ErrorSerializer,
             adapter: :json_api
-          })
-        expected_response_document =
-          { :errors =>
-            [
-              { :source => { :pointer => '/data/attributes/title' }, :detail => 'must be amazing' }
-            ]
+          }
+        )
+        expected_response_document = {
+          :errors => [
+            { :source => { :pointer => '/data/attributes/title' }, :detail => 'must be amazing' }
+          ]
         }
         assert_equal serializable_resource.as_json(options), expected_response_document
       end