Fix nil error when trying to fetch key for signature verification (mastodon#17747)

Author: Gargron

app/helpers/jsonld_helper.rb

@@ -54,7 +54,7 @@ def supported_context?(json)
   end
 
   def unsupported_uri_scheme?(uri)
-    !uri.start_with?('http://', 'https://')
+    uri.nil? || !uri.start_with?('http://', 'https://')
   end
 
   def invalid_origin?(url)