chore: add empty requests constant #4619

Job	Run time
doctest	1m 37s
test ubuntu-latest stable --no-default-features	1m 38s
check no_std	24s
test ubuntu-latest stable	1m 41s
test ubuntu-latest stable --all-features	2m 25s
clippy	47s
feature-checks	3m 54s
test ubuntu-latest nightly --no-default-features	1m 30s
wasm-unknown	56s
test ubuntu-latest nightly	1m 24s
wasm-wasi	1m 23s
test ubuntu-latest nightly --all-features	2m 0s
test ubuntu-latest 1.79 --no-default-features	45s
cfg-check	25s
test ubuntu-latest 1.79	47s
test windows-latest stable --no-default-features	3m 46s
docs	1m 8s
test windows-latest stable	3m 26s
test windows-latest stable --all-features	4m 29s
fmt	13s
test windows-latest nightly --no-default-features	3m 15s
test windows-latest nightly	3m 0s
test windows-latest nightly --all-features	5m 2s
test windows-latest 1.79 --no-default-features	2m 3s
test windows-latest 1.79	3m 57s
	51m 55s

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