There are two connection points on the niche. The outer connection lug is the bonding lug. Since the niche contains metal, it has to be bonded. This bond lug is physically/electrically connected to the inside lug and to the metal strip in the niche. The metal strip creates a water bond.
The purpose of the inside lug is to create a connection point for a supplemental equipment grounding conductor between the niche, the light and the junction box. The supplemental grounding conductor can be metal conduit or a #8 green insulated grounding conductor in solid or stranded.
If you have a listed low voltage light that does not require a ground, the supplemental grounding conductor is not required. The supplemental equipment grounding conductor is also referred to as a “bonding jumper” because it bonds the niche to the junction box, but its purpose is grounding, not bonding.
The installation manual says to use the ground wire and it doesn’t seem to mention an exclusion for low voltage.
The code is confusing at best. In my opinion, the internal grounding conductor is not required for a listed low voltage light that does not require a ground.
Some inspectors might require it if they think that it’s required. So, you’re probably safer to include it. It will also give you the option to change to a 120 volt light later.
It looks like you already have the #8 wire in the conduit anyway. So, might as well use it.
Also, the internal connection needs to be sealed with an approved potting compound to isolate the connection from the water.
The light fixture itself is grounded
If it's a low voltage light, it won't usually have a ground. Are you sure that it has a ground?
If the light has a ground, then you do need the internal supplemental #8 green insulated equipment grounding conductor (bonding jumper) from the niche to the junction box.
If the conduit from the niche to the junction box is metal, the ground wire is not required in either case because the metal conduit is the grounding conductor.
Also, if the junction box has a bond lug on the bottom of the metal base, you should bond it with a bond wire to the bonding grid.