OpenGL: Fix wrong default texture for global uniforms of type sampler2DArray

[dsnopek]

Fixes #101938

There were two problem in this code:

1. It always added a texture, even if it wasn't set on either value or override
2. It assumed an unset value would be Variant::NIL, however, it looks like it's Variant::RID but if it's not set it'll be an empty RID (ie RID()).

With this PR, if you open the MRP from #101938, it will no longer spam the error every frame

[clayjohn]

Is the same issue present when using the mobile or Forward+ renderer? the same code is present there as well