Fix trying to fetch key from empty URI when verifying HTTP signature (#16100)

app/helpers/jsonld_helper.rb

@@ -67,7 +67,7 @@ module JsonLdHelper
     unless id
       json = fetch_resource_without_id_validation(uri, on_behalf_of)
 
-      return unless json
+      return if !json.is_a?(Hash) || unsupported_uri_scheme?(json['id'])
 
       uri = json['id']
     end

app/services/activitypub/fetch_remote_key_service.rb

@@ -5,6 +5,8 @@ class ActivityPub::FetchRemoteKeyService < BaseService
 
   # Returns account that owns the key
   def call(uri, id: true, prefetched_body: nil)
+    return if uri.blank?
+
     if prefetched_body.nil?
       if id
         @json = fetch_resource_without_id_validation(uri)