Don't crash in development if the Rails master key is missing

Oops, right, this meta tag that runs on all pages currently crashes if
we can't read the credentials file!

Instead, let's just allow this value to be `nil` if not present.

app/helpers/application_helper.rb

@@ -101,7 +101,8 @@ module ApplicationHelper
 
   def impress_2020_meta_tags
     origin = Rails.configuration.impress_2020_origin
-    support_secret = Rails.application.credentials.impress_2020.support_secret
+    support_secret = Rails.application.credentials.dig(
+      :impress_2020, :support_secret)
 
     capture do
       concat tag("meta", name: "impress-2020-origin", content: origin)