Fix vk_imgui crash by allowing zero-sized uniform buffers.
Fix vk_imgui crash by allowing zero-sized uniform buffers.

The vk_imgui sample regressed in December or so, after some changes in
how Filament manages uniform buffers. This commit makes the Vulkan
driver more consistent with the GL driver, which does a check for
zero-sized buffers.

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament