Ensure that ActiveStorage.analyzers doesn't contain nil

When setting `config.active_storage.variant_processor = nil`, there will be neither vips nor mini_magick.
Not error will be raised as nothing is autoloaded, so `analysers` will be nil. This later blows up
when trying to analyse a blob.

Author: Earlopain

activestorage/lib/active_storage/engine.rb

@@ -102,7 +102,7 @@ class Engine < Rails::Engine # :nodoc:
               ]
             end
 
-          ActiveStorage.analyzers = [analyzer].concat(app.config.active_storage.analyzers || [])
+          ActiveStorage.analyzers = [analyzer].compact.concat(app.config.active_storage.analyzers || [])
           ActiveStorage.variant_transformer = transformer
         rescue LoadError => error
           case error.message

railties/test/application/configuration_test.rb

@@ -3931,6 +3931,14 @@ class Post < ActiveRecord::Base
       assert_equal :vips, ActiveStorage.variant_processor
     end
 
+    test "ActiveStorage.analyzers doesn't contain nil when variant_processor = nil" do
+      add_to_config "config.active_storage.variant_processor = nil"
+
+      app "development"
+
+      assert_not_includes ActiveStorage.analyzers, nil
+    end
+
     test "ActiveStorage.supported_image_processing_methods can be configured via config.active_storage.supported_image_processing_methods" do
       remove_from_config '.*config\.load_defaults.*\n'