TextureRD: Ignore invalid free error and keep rid unchanged when replacing rd texture

[beicause]

Fixes #95273. Fixes #85989.

• Don't change TextureRD's rid when assigned an empty texture_rd_rid, Instead, repalce its texture by a placeholder texture.
• Check if rd texture is valid to avoid printing errors when users have freed texture_rd_rid of TextureRD and replace the rd texture.

[beicause]

Looking back, I realized the way I used to check if the rd texture is valid is wrong.
I think it makes more sense to ignore the invalid free errors because we already do this in texture_free:

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

Lines 60 to 67 in 2d113cc

	void TextureStorage::Texture::cleanup() {
	if (RD::get_singleton()->texture_is_valid(rd_texture_srgb)) {
	//erase this first, as it's a dependency of the one below
	RD::get_singleton()->free(rd_texture_srgb);
	}
	if (RD::get_singleton()->texture_is_valid(rd_texture)) {
	RD::get_singleton()->free(rd_texture);
	}