Avoid using empty descriptor set layouts with libraries

Avoid using empty descriptor set layouts with libraries

When creating pipeline libraries, the different pipeline layouts need to
be combined on a union of pipeline layouts, and sets not used for a
particular library can be VK_NULL_HANDLE when those pipeline layouts are
created with VK_PIPELINE_LAYOUT_CREATE_INDEPENDENT_SETS_BIT_EXT.

Some of the pipeline library tests are using empty descriptor sets
instead of null handles for this case, breaking some drivers.

Affected tests:
dEQP-VK.pipeline.pipeline_library.graphics_library.*

VK-GL-CTS issue: 3584
Components: Vulkan

Change-Id: I6ee66f5d350bf586ac680f222ac5e426cb36a2e5