Probably no studies have been done on this interaction. Active ingredients in valerian have been shown to bind to the benzodiazepine receptor in the brain.
This is the receptor benzodiazepines (e.g. Valium, Xanax, Ativan etc.) bind to.
Patients are commonly being prescribed Zoloft plus Xanax or some other benzodiazepine together with no ill effects, so I see no problem with Zoloft & valerian.