Fix several Material texture parameter updates

[bitsawer]

• Fixes Making a material unique with emission enabled will result in incorrect editor colors #60849
• Fixes resource_local_to_scene causes emission materials to break visuals of MeshInstance3D half the time #83190
• Fixes When no emission_texture is set on a BaseMaterial3D, it defaults to white instead of black after duplicating it #84139

Fixes the issues when assigning or initializing a null/default texture to certain material properties doesn't properly update the material state. This can happen in normal use when updating textures in code, but usually this is more noticeable when duplicating a material either explicitly by calling duplicate() or indirectly during instantiation if the material has Local To Scene enabled. This bug causes certain empty textures in a material to be initialized incorrectly. Seems like at least emission, back light and height mapping textures are most affected as they use hint_default_black fallback.

I also noticed possible issues in particle process material, so this fix may also affect particle system behavior in certain situations. In normal materials this bug is usually obvious and can be worked around as discussed in the issue reports, but with particle systems the change in behavior might not be immediately clear.

The basic problem is that calling RenderingServer::material_set_param() with a Variant::Type::NIL parameter value will result in different behavior compared to using a zero/null RID. Only a NIL Variant will clear the internal material parameter inside the MaterialStorage:

godot/servers/rendering/renderer_rd/storage_rd/material_storage.cpp

Lines 2142 to 2147 in 6afd320

	if (p_value.get_type() == Variant::NIL) {
	material->params.erase(p_param);
	} else {
	ERR_FAIL_COND(p_value.get_type() == Variant::OBJECT); //object not allowed
	material->params[p_param] = p_value;
	}

Many places already correctly use the NIL Variant, so I changed all obvious places I could find in this PR. Another alternative is to change MaterialStorage::material_set_param() to clear the material (texture) property also when using an empty RID. However, as that API is exposed to scripting it would change existing behavior, although I doubt many people are relying on that behavior and if they are their code is probably buggy in a similar way.

[clayjohn]

Looks great! Seems like we caught this for SkyMaterials a long time ago and just never checked the other places where this pattern is used (#75469 (review))

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.4. (Omitting some methods introduced by PRs which were not and will not be cherry-picked, such as the particles refactor).