asset: change all-zero hash to empty leaf #3630

Job	Run time
Docker build check	3m 45s
cross compilation	5m 27s
Format check	19s
Compilation check	51s
run itests	9m 44s
Lint check	2m 53s
run itests postgres	17m 43s
migration version check	10s
RPC check	1m 14s
go mod check	20s
sample configuration check	42s
run LiT tests	14m 1s
test vector check	58s
Sqlc check	2m 40s
run unit tests (unit-race)	12m 35s
run unit tests (unit-cover)	3m 18s
run unit tests (unit dbbackend=postgres)	6m 0s
finish	5s
	1h 22m 45s

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