Refactor blank_allowed

If passed a nil in the personalisation, the Notify API will raise an
error.

The `blank_allowed` method lets developers opt-in to allow a blank
value, here 'blank' probably means `nil`.

As v2 no longer relies on the `Personalisation` class to handle
personalisations we want to remove the class, but this method still
relies on it.

We want to keep the method in-case it is used.

In v2, we really only need to know when a developer is opting in by
calling the method and then convert nil into an empty string.

lib/mail/notify.rb

@@ -5,7 +5,6 @@
 require "mail/notify/version"
 require "mail/notify/railtie" if defined? Rails
 require "mail/notify/delivery_method"
-require "mail/notify/personalisation"
 require "mail/notify/mailer"
 require "mail/notify/message"
 require "mail/notify/mail_notify_previews_controller"

lib/mail/notify/mailer.rb

@@ -91,7 +91,7 @@ def view_mail(template_id, options)
       # allows blank personalisation options
 
       def blank_allowed(value)
-        value.presence || Personalisation::BLANK
+        value.to_s
       end
     end
   end

spec/mail/notify/mailer_spec.rb

@@ -211,9 +211,15 @@
       end
     end
 
-    context "when given a blank value" do
+    context "when given an empty string" do
       it "returns explicitly blank personalisation" do
-        expect(TestMailer.new.blank_allowed("")).to be Mail::Notify::Personalisation::BLANK
+        expect(TestMailer.new.blank_allowed("")).to eq ""
+      end
+    end
+
+    context "when given nil" do
+      it "returns an empty string" do
+        expect(TestMailer.new.blank_allowed(nil)).to eq ""
       end
     end
   end