eth/gasprice: fix wrong gas price with empty blocks #1795

Job	Run time
tests (A-B tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFinOrg/...	1m 25s
tests (C-[a-m] tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFin...	1m 44s
tests (C-[n-o] tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFin...	7m 46s
tests (C-[p-z] tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFin...	36s
tests (D-I tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFinOrg/...	2m 50s
tests (J-N tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFinOrg/...	2m 7s
tests (O-R tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFinOrg/...	1m 25s
tests (S tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFinOrg/XD...	11m 56s
tests (T-Z tests, go run build/ci.go test -coverage $(go list ./... \| grep "github.com/XinFinOrg/...	1m 50s
tag_build	0s
devnet_build_push	0s
rpcnode_terraform_apply	0s
devnet_terraform_apply	0s
devnet_dev-upgrade_node	0s
devnet_send_notification	0s
mainnet_dev-upgrade_node	0s
testnet_dev-upgrade_node	0s
	31m 39s

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