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

[dsnopek]

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

Ah, yeah, looking at the code, it certainly looks like it has the same problem.

When I test it with the Mobile or Forward+ renderer, I don't get any errors (I really only noticed the problem under OpenGL because of the error spam). I tried enabling the Vulkan validation layers, and, unfortunately, I get endless error spam from them even with the global uniform removed - so most of the messages are unrelated to this issue. But that makes it hard to tell if there is or isn't an error message about this in there. Just scrolling through it, I don't see an error message that seems to be related to this issue?

Adding some print statements to the relevant code, seems to show that value is Variant::RID even when empty - whereas override is actually Variant::NIL. While that's a little different than what I was seeing for OpenGL, it does look like the same issue basically applies, and can be solved with the same code.

So, I'll update my PR to address this for Mobile and Forward+ as well!