pkg/aflow/flow/patching: harden against empty generated patches #19158

Job	Run time
race_dashboard	1m 13s
aux	1m 1s
build	1m 14s
race	1m 13s
arch (presubmit_arch_freebsd)	1m 1s
arch (presubmit_arch_darwin)	1m 1s
old	1m 5s
arch (presubmit_arch_linux)	1m 0s
arch (presubmit_arch_netbsd)	1m 1s
arch (presubmit_arch_openbsd)	1m 1s
gvisor	1m 1s
dashboard	1m 9s
arch (presubmit_arch_windows)	1m 1s
arch (presubmit_arch_executor)	1m 1s
	15m 2s

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