Ignore deleted files in Active Storage migration

Files might be missing for whatever reason or records might not point to
any files; in these edge cases, we were getting an exception.

lib/tasks/active_storage.rake

@@ -40,7 +40,11 @@ namespace :active_storage do
 
         model.find_each.each do |instance|
           attachments.each do |attachment|
-            next if instance.send(attachment).path.blank?
+            source = instance.send(attachment).path
+
+            next if source.blank?
+
+            file = File.read(source) if File.exist?(source)
 
             connection.exec_prepared(
               statement_name, [
@@ -48,7 +52,7 @@ namespace :active_storage do
                 instance.send("#{attachment}_file_name"),
                 instance.send("#{attachment}_content_type"),
                 instance.send("#{attachment}_file_size"),
-                Digest::MD5.base64digest(File.read(instance.send(attachment).path)),
+                file && Digest::MD5.base64digest(file) || SecureRandom.hex(32),
                 instance.updated_at.iso8601,
                 "storage_#{attachment}",
                 model.name,
@@ -65,9 +69,11 @@ namespace :active_storage do
       name = attachment.name.delete_prefix("storage_")
       source = attachment.record.send(name).path
 
-      FileUtils.mkdir_p(File.dirname(dest))
-      logger.info "Copying #{source} to #{dest}"
-      FileUtils.cp(source, dest)
+      if source && File.exist?(source)
+        FileUtils.mkdir_p(File.dirname(dest))
+        logger.info "Copying #{source} to #{dest}"
+        FileUtils.cp(source, dest)
+      end
     end
   end
 end