tu: Fix binding NULL descriptor sets
This fixes the new test dEQP-VK.pipeline.pipeline_library.graphics_library.misc.other.null_descriptor_set_in_monolithic_pipeline.
Fixes: e9f5de11 ("tu: Initial implementation of VK_EXT_graphics_pipeline_library")
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This fixes the new test dEQP-VK.pipeline.pipeline_library.graphics_library.misc.other.null_descriptor_set_in_monolithic_pipeline.
Fixes: e9f5de11 ("tu: Initial implementation of VK_EXT_graphics_pipeline_library")