query.js: do not raise if the sort_on_option list is empty

Fix the error

Error: TypeError: Cannot read property '0' of undefined

when synchronizing officejs apps (see https://nexedijs.erp5.net/#/bug_module/20190227-23F861F)

I confirm the synchro works again by applying this page locally.

I couldn't find where in the code (neither in jio and in ERP5) the sort_on parameter set to []. Maybe it's done dynamically somewhere (in replicate storage ?).