Allow for "empty" bindings, which are valid but represent when no
object is bound.

BUG=angleproject:2052

Change-Id: I0a41d1f0db3f5736e9e8f8ca3a74f41b748fd2d2
Reviewed-on: https://chromium-review.googlesource.com/659225
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>