Add null checks to xnnpack flatbuffer #1921

Job	Run time
test-vulkan / test-backend-linux (vulkan, models) / linux-job	20m 54s
test-vulkan / test-backend-linux (vulkan, operators) / linux-job	22m 2s
test-vulkan / test-backend-macos	-1s
test-vulkan / package-golden-artifacts	3m 31s
	46m 26s

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