Clarify what first_block `None` means

10 jobs for 11269 in 24 minutes and 48 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #305493
linux-docker allowed to fail
cargo-audit

00:00:31

passed #305488
linux-docker
cargo-check 0 3

00:02:34

passed #305489
linux-docker
cargo-check 1 3

00:01:15

passed #305490
linux-docker
cargo-check 2 3

00:01:15

passed #305492
linux-docker
cargo-check-benches

00:04:44

passed #305491
linux-docker
cargo-check-evmbin

00:01:58

passed #305494
linux-docker
validate-chainspecs

00:02:24

 
  Build
passed #305497
linux-docker
build-android

00:20:02

passed #305495
linux-docker
test-cpp

00:03:44

passed #305496
linux-docker
test-linux

00:17:02