if cookie is tampered with then nil is returned [ci skip]

if the given key is not found then verifier does raise
`ActiveSupport::MessageVerifier::InvalidSignature` exception
but this exception is resuced and finally nil is returned.
Merge pull request #9916 from neerajdotname/no-exception-when-cookie-is-tampered-with

nil is retuned if cookie is tampered with [ci skip]