pkg/aflow/flow/patching: harden against empty generated patches #19165

Job	Run time
aux	3m 58s
race	9m 34s
gvisor	4m 5s
arch (presubmit_arch_freebsd)	6m 2s
arch (presubmit_arch_openbsd)	3m 58s
dashboard	2m 8s
old	7m 21s
arch (presubmit_arch_linux)	11m 4s
race_dashboard	3m 21s
arch (presubmit_arch_executor)	3m 16s
arch (presubmit_arch_darwin)	3m 50s
build	3m 26s
arch (presubmit_arch_windows)	3m 35s
arch (presubmit_arch_netbsd)	3m 50s
	1h 9m 28s

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