VertexProgram and PixelProgram always have a shader.

Bug chromium:805881

Change-Id: I0a27b4e7a0bc0ae591d89eb295e215bd421416fb
Reviewed-on: https://swiftshader-review.googlesource.com/16749Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>