Fix handling of malformed ActivityPub payloads when URIs are nil (#7370)

* Fix handling of malformed ActivityPub payloads when URIs are nil

* Gracefully handle JSON-LD canonicalization failures

app/services/activitypub/process_collection_service.rb

@@ -45,5 +45,8 @@ class ActivityPub::ProcessCollectionService < BaseService
 
   def verify_account!
     @account = ActivityPub::LinkedDataSignature.new(@json).verify_account!
+  rescue JSON::LD::JsonLdError => e
+    Rails.logger.debug "Could not verify LD-Signature for #{value_or_id(@json['actor'])}: #{e.message}"
+    nil
   end
 end