The spec states:
  Shader stages including vertex shaders, fragment shaders, and compute
  shaders...A single program object can contain all of these shaders, or
  any subset thereof.

This change allows shader programs without a fragment shader. The
biggest driver of this change is dEQP since a large number of tests
create shader programs without a fragment shader.

Bug: angleproject:3803
Test: dEQP-GLES31.functional.program_interface_query
Change-Id: Id6cb098c62a1489a14b9ec1b31bd4cd59f655e49
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1752010
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>