Active Storage: Blob creation shouldn't crash if no service selected

#41653 noted an issue where if there's service configured (`config.active_storage.service` is commented out), Blob creation via direct upload crashes:

```
Started POST "/rails/active_storage/direct_uploads" for ::1 at 2021-03-09 13:02:57 -0800
Processing by ActiveStorage::DirectUploadsController#create as JSON
  Parameters: {"blob"=>{"filename"=>"banana.jpg", "content_type"=>"image/jpeg", "byte_size"=>577085, "checksum"=>"W/vo/JqBNmJHMCaL+PRlBQ=="}, "direct_upload"=>{"blob"=>{"filename"=>"banana.jpg", "content_type"=>"image/jpeg", "byte_size"=>577085, "checksum"=>"W/vo/JqBNmJHMCaL+PRlBQ=="}}}
Completed 500 Internal Server Error in 12ms (ActiveRecord: 3.3ms | Allocations: 5864)

NoMethodError (undefined method `name' for nil:NilClass):

activestorage (6.1.3) app/models/active_storage/blob.rb:52:in `block in <class:Blob>'
activesupport (6.1.3) lib/active_support/callbacks.rb:427:in `instance_exec'
```

This PR fixes that crash. Blob creation will still fail, but with a more informative error about a `service_name` being required.

activestorage/app/models/active_storage/blob.rb

@@ -49,7 +49,7 @@ class ActiveStorage::Blob < ActiveStorage::Record
   scope :unattached, -> { where.missing(:attachments) }
 
   after_initialize do
-    self.service_name ||= self.class.service.name
+    self.service_name ||= self.class.service&.name
   end
 
   after_update_commit :update_service_metadata, if: :content_type_previously_changed?

activestorage/test/models/blob_test.rb

@@ -252,6 +252,14 @@ class ActiveStorage::BlobTest < ActiveSupport::TestCase
     end
   end
 
+  test "doesn't create a valid blob if service setting is nil" do
+    with_service(nil) do
+      assert_raises(ActiveRecord::RecordInvalid) do
+        create_blob(filename: "funky.jpg")
+      end
+    end
+  end
+
   test "invalidates record when provided service_name is invalid" do
     blob = create_blob(filename: "funky.jpg")
     blob.update(service_name: :unknown)

activestorage/test/test_helper.rb

@@ -102,7 +102,7 @@ def fixture_file_upload(filename)
 
     def with_service(service_name)
       previous_service = ActiveStorage::Blob.service
-      ActiveStorage::Blob.service = ActiveStorage::Blob.services.fetch(service_name)
+      ActiveStorage::Blob.service = service_name ? ActiveStorage::Blob.services.fetch(service_name) : nil
 
       yield
     ensure