Gracefully degrade when ActionMailer::Base#params is nil

Prior to this change, access to `params` on `ActionMailer::Base`
instances prior to being decorated by `ActionMailer::Parameterized.with`
calls results in a `NoMethodError`:

```
Error:
ParameterizedTest#test_degrade_gracefully_when_.with_is_not_called:
NoMethodError: undefined method `[]' for nil:NilClass

  before_action { @Inviter, @invitee = params[:inviter], params[:invitee] }
                                             ^^^^^^^^^^
```

This change modifies the `attr_accessor :params` to be an `attr_writer`
paired with a `params` method that assigns `@params` to an empty `Hash`
whenever it's accessed without being otherwise initialized.

actionmailer/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Default `ActionMailer::Parameterized#params` to an empty `Hash`
+
+    *Sean Doyle*
+
 *   `assert_emails` now returns the emails that were sent.
 
     This makes it easier to do further analysis on those emails:

actionmailer/lib/action_mailer/parameterized.rb

@@ -88,7 +88,11 @@ module Parameterized
     extend ActiveSupport::Concern
 
     included do
-      attr_accessor :params
+      attr_writer :params
+
+      def params
+        @params ||= {}
+      end
     end
 
     module ClassMethods

actionmailer/test/parameterized_test.rb

@@ -37,6 +37,13 @@ class DummyDeliveryJob < ActionMailer::MailDeliveryJob
     assert_equal("So says david@basecamp.com", @mail.body.encoded)
   end
 
+  test "degrade gracefully when .with is not called" do
+    @mail = ParamsMailer.invitation
+
+    assert_nil(@mail.to)
+    assert_nil(@mail.from)
+  end
+
   test "enqueue the email with params" do
     args = [
       "ParamsMailer",