Add check before removing old params_yaml column

Check for content in InfoRequestEvent#params_yaml column.

If a re-user skips 0.42.0.0 upgrade notes then they could lose data. Add
a check to ensure we prevent this.

db/migrate/20230127132719_remove_info_request_event_params_yaml.rb

@@ -1,5 +1,22 @@
 class RemoveInfoRequestEventParamsYaml < ActiveRecord::Migration[7.0]
-  def change
-    remove_column :info_request_events, :params_yaml, :text
+  def up
+    if InfoRequestEvent.where(params: nil).any?
+      raise <<~TXT
+        We can't run the RemoveInfoRequestEventParamsYaml database migrations.
+
+        We have dectected InfoRequestEvent objects which haven't been migrated
+        to the JSONB params column.
+
+        Please deploy Alaveteli 0.42.0.0 and run the upgrade tasks:
+        https://github.com/mysociety/alaveteli/blob/0.42.0.0/doc/CHANGES.md#upgrade-notes
+
+      TXT
+    end
+
+    remove_column :info_request_events, :params_yaml
+  end
+
+  def down
+    add_column :info_request_events, :params_yaml, :text
   end
 end