Fix image entity crash on setSampler

[ksuprynowicz]

Fixes #1970

[JulianGro]

Am I right in the assumption that we try to render an image entity without knowing if the texture is loaded yet, and that is what is causing getGPUTexture() to return nullptr?
I am assuming that the texture is loaded by something else, while we are building the batch, and it is easier to just try and use it rather than only trying to render things that are finished loading?
I can see that we set setResourceTexture()'s texture to nullptr quite often in our code, so it must be intentional, right?

[HifiExperiments]

great catch - sorry for not noticing this. I might suggest a simpler fix of just early exiting:

Suggested change

	auto gpuTexture = _texture->getGPUTexture();
	if (gpuTexture != nullptr) {
	_texture->getGPUTexture()->setSampler(sampler);
	}
	// It's ok to pass nullptr to setResourceTexture.
	batch->setResourceTexture(0, _texture->getGPUTexture());
	auto gpuTexture = _texture->getGPUTexture();
	if (!gpuTexture) {
	return;
	}
	gpuTexture->setSampler(sampler);
	batch->setResourceTexture(0, gpuTexture);

while calling setResourceTexture with nullptr is totally safe, it means we're still doing a draw call with an unset texture, which probably isn't desirable.

this is also closer to the intention of the early exit on line 142. in fact, a lot of that logic above should probably be brought into this if (pipelineType == Pipeline::SIMPLE) case, since procedural + material paths don't actually use _texture, but no one is really using those anyways...