pkg/aflow/flow/patching: harden against empty generated patches #19156

Job	Run time
race	7m 14s
dashboard	2m 8s
arch (presubmit_arch_netbsd)	4m 1s
build	3m 13s
old	6m 44s
arch (presubmit_arch_openbsd)	3m 25s
gvisor	4m 5s
race_dashboard	3m 54s
arch (presubmit_arch_windows)	3m 21s
arch (presubmit_arch_linux)	7m 6s
arch (presubmit_arch_darwin)	3m 45s
aux	4m 12s
arch (presubmit_arch_freebsd)	6m 29s
arch (presubmit_arch_executor)	3m 24s
	1h 3m 1s

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