Address rubocop warnings

lib/active_model/serializer/collection_serializer.rb

@@ -14,7 +14,7 @@ module ActiveModel
           serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
           serializer_class = options.fetch(:serializer) { serializer_context_class.serializer_for(resource) }
 
-          if serializer_class.nil?
+          if serializer_class.nil? # rubocop:disable Style/GuardClause
             fail NoSerializerError, "No serializer found for resource: #{resource.inspect}"
           else
             serializer_class.new(resource, options.except(:serializer))

lib/active_model_serializers/adapter.rb

@@ -1,7 +1,7 @@
 module ActiveModelSerializers
   module Adapter
     UnknownAdapterError = Class.new(ArgumentError)
-    ADAPTER_MAP = {}.freeze
+    ADAPTER_MAP = {} # rubocop:disable Style/MutableConstant
     private_constant :ADAPTER_MAP if defined?(private_constant)
     require 'active_model_serializers/adapter/fragment_cache'
     require 'active_model_serializers/adapter/cached_serializer'

lib/active_model_serializers/adapter/json_api/relationship.rb

@@ -38,8 +38,7 @@ module ActiveModelSerializers
         def data_for(serializer, options)
           if serializer.respond_to?(:each)
             serializer.map { |s| ResourceIdentifier.new(s).as_json }
-          else
-            if options[:virtual_value]
+          elsif options[:virtual_value]
             options[:virtual_value]
           elsif serializer && serializer.object
             ResourceIdentifier.new(serializer).as_json
@@ -49,4 +48,3 @@ module ActiveModelSerializers
     end
   end
 end
-end