Fixed a crash in WithVoxelBarrel

Fixes the problem #6914 revealed by https://github.com/OpenRA/OpenRA/pull/6710.
Hmm, I was under the impression these `First()` calls make it non-optional https://github.com/Mailaender/OpenRA/blob/voxel-barrel-requires/OpenRA.Mods.RA/Render/WithVoxelBarrel.cs#L36.