Use dig for safe nested array navigation

This fixes a potential issue trying to navigate a double-nested array by
using `dig` instead.

Sentry-Issue: 6855749667

Author: thomasleese

app/views/vaccines/index.html.erb

@@ -30,7 +30,7 @@
                   <%= batch.name %>
                 </span>
 
-                <% if batch.id == @todays_batch_id_by_programme_and_vaccine_methods[vaccine.programme][vaccine.method] %>
+                <% if batch.id == @todays_batch_id_by_programme_and_vaccine_methods.dig(vaccine.programme, vaccine.method) %>
                   <br>
                   <span class="nhsuk-caption-m">
                     (Your default)