syz-cluster: support empty DocLink in the email template #17043

Job	Run time
aux	53s
build	53s
race	1s
dashboard	53s
race_dashboard	1s
arch (presubmit_arch_openbsd)	53s
old	58s
arch (presubmit_arch_freebsd)	47s
arch (presubmit_arch_linux)	1m 0s
arch (presubmit_arch_darwin)	54s
arch (presubmit_arch_netbsd)	55s
arch (presubmit_arch_windows)	52s
arch (presubmit_arch_executor)	57s
gvisor	50s
	10m 47s

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