Do not crash if remote custom emoji does not define updated date (fixes #8376) (#8377)

app/lib/activitypub/activity/create.rb

@@ -107,7 +107,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     updated   = tag['updated']
     emoji     = CustomEmoji.find_by(shortcode: shortcode, domain: @account.domain)
 
-    return unless emoji.nil? || emoji.updated_at >= updated
+    return unless emoji.nil? || image_url != emoji.image_remote_url || (updated && emoji.updated_at >= updated)
 
     emoji ||= CustomEmoji.new(domain: @account.domain, shortcode: shortcode, uri: uri)
     emoji.image_remote_url = image_url

app/services/activitypub/process_account_service.rb

@@ -226,7 +226,7 @@ class ActivityPub::ProcessAccountService < BaseService
     updated   = tag['updated']
     emoji     = CustomEmoji.find_by(shortcode: shortcode, domain: @account.domain)
 
-    return unless emoji.nil? || emoji.updated_at >= updated
+    return unless emoji.nil? || image_url != emoji.image_remote_url || (updated && emoji.updated_at >= updated)
 
     emoji ||= CustomEmoji.new(domain: @account.domain, shortcode: shortcode, uri: uri)
     emoji.image_remote_url = image_url