zink: don't run update_so_info if shader has no outputs
this probably means all outputs were eliminated by the linker, but in any case it's not going to do anything
fixes spec@glsl-1.50@execution@interface-blocks-api-access-members
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this probably means all outputs were eliminated by the linker, but in any case it's not going to do anything
fixes spec@glsl-1.50@execution@interface-blocks-api-access-members