ListBox: ignore non string request keys

This can happen when request is used as a cache storage, especially
after eab00f14 (deferred_style: Properly keep request parameters,
2021-03-10)
4 jobs for e2e-erp5 in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-TestRunner1

06:19:02

failed ERP5.UnitTest-Zope2-ForTestRunner1

06:06:16

failed ERP5.UnitTest-TestRunner1

05:09:14

failed ERP5.UnitTest-Zope2-ForTestRunner1

05:11:33