We were expecting an uninitialized texture to contain black. However
the contents of the texture data may be undefined.

Discovered when working with the Vulkan command stream refactor.

Bug: angleproject:4029
Change-Id: Ia5d24707746819b116ac0053fabb48033574569a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2017978Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>