Connecting DIODE in relay circuit
The purpose of adding a reverse biased diode to a mechanical relay is to prevent a reverse voltage potential appearing at the driver circuit when the magnetic field around the coil collapses, which happens when the relay coil is suddenly disconnected.

A solid state relay does not use magnetic fields.  Instead it uses light:



So there is no collapsing magnetic field to generate a reverse voltage potential.  It follows no reverse biased diode (or flyback diode) is necessary.