Paulownia soundboards are very soft, so an accidental knock can leave a small dent. Never fill the hollow with chemical glue mixed with wood dust — it will affect the instrument’s tone.
Instead, drip clean water into the dent and let the wood absorb it slowly so the compressed paulownia springs back. If it does not return fully, fill the remaining hollow with animal glue mixed with paulownia dust, sand it smooth, then colour it with shellac solution. Once animal glue dries it leaves natural voids in the cured layer, so it does not block the transmission of sound. If you are not concerned about a minor cosmetic flaw, no repair is needed at all — it does not affect the tone.