[1.13.0 backport] finalization: Skip tree route calculation if no forks present #1723

Job	Run time
set-image	11s
check-runtime-migration (asset-hub-westend)	4m 18s
check-runtime-migration (asset-hub-rococo)	22m 50s
check-runtime-migration (bridge-hub-westend)	4m 5s
check-runtime-migration (bridge-hub-rococo)	4m 21s
check-runtime-migration (contracts-rococo)	10m 24s
check-runtime-migration (collectives-westend)	4m 10s
check-runtime-migration (coretime-rococo)	4m 24s
	54m 43s

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