Test Runner: Allow empty sharding parameters.
Test Runner: Allow empty sharding parameters.

The prior check was incorrectly validating a shard index of zero
and a shard count of one as inconsistent.

Bug: angleproject:3162
Change-Id: I63184c16e82f08080b275672a43e6dacf291560d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2419045
Reviewed-by: Jonah Ryan-Davis <682bdb9e7d8ba359ef169b8db374e7a99d394a46@google.com>
Commit-Queue: Jonah Ryan-Davis <682bdb9e7d8ba359ef169b8db374e7a99d394a46@google.com>

ppy/angle,ppy/angle,ppy/angle,ppy/angle