We were assuming at least one variable, when some shaders could
legitimately use zero. This was causing assertions in the
dEQP shader.function tests.

BUG=angle:855

Change-Id: I7e4aa52a1c6ba98d4a396f4c642816ff95115fd5
Reviewed-on: https://chromium-review.googlesource.com/240144Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>