The trace of any electromagnetic energy-stress tensor is traceless. The trace of an energy-stress tensor of massless particles, is a sum of electromagnetic energy stress tensor and gravitational energy-stress tensor is also traceless. However, there is a difference because the sum of two massless particles can be massive [1].

Reference
1. Completing Einstein’s Proof of E = mc2, Progress in Phys., Vol. 4, 14-18 (2006).