kong_api_authz: Fix error when no JWT token is provided

When no token is provided with the request, the execution fail the attempt to index the field 'payload' (a nil value).
In case the subject from the JWT token is not be defined or there's no token, 'anonymous' will be used instead.

Fixes open-policy-agent#146

kong_api_authz/src/kong/plugins/opa/access.lua

@@ -75,7 +75,7 @@ function _M.execute(conf)
     kong.log.debug(interp("Access allowed to ${method} ${path} for user ${subject}", {
         method = input.method,
         path = input.path,
-        subject = token.payload.sub
+        subject = (token.payload and token.payload.sub or 'anonymous')
     }))
 end