pkg/aflow/flow/patching: harden against empty generated patches #19159

Job	Run time
dashboard	2m 15s
race	7m 21s
gvisor	4m 1s
arch (presubmit_arch_freebsd)	6m 6s
build	4m 9s
arch (presubmit_arch_netbsd)	3m 51s
aux	3m 37s
arch (presubmit_arch_executor)	2m 58s
arch (presubmit_arch_openbsd)	3m 48s
race_dashboard	3m 44s
arch (presubmit_arch_darwin)	3m 47s
arch (presubmit_arch_windows)	3m 44s
old	7m 15s
arch (presubmit_arch_linux)	10m 46s
	1h 7m 22s

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