Do I need to add a ground conductor when replacing a wall light fixture?
If the fixture has the 'class II' (double-insulaton) logo on:
,
 you don't need to add a ground, also if the circuit is GFCI protected you don't need a ground, if light is 'out of reach' lack of ground may not be a problem (as long as you turn the breaker off when changhing a bulb).

In general, if all the fixture casing is made of insulating materials, ground is not required.