Fix 5288: Doesn't POST if attestations is empty. #15478

Job	Run time
beacon-chain-tests	20m 9s
check-code	10m 35s
check-msrv	4m 27s
cargo-udeps	6m 40s
cli-check	18m 22s
compile-with-beta-compiler	13m 44s
debug-tests-ubuntu	14m 48s
dockerfile-ubuntu	13m 42s
doppelganger-protection-test	22m 45s
ef-tests-ubuntu	13m 29s
execution-engine-integration-ubuntu	14m 57s
merge-transition-ubuntu	30m 33s
eth1-simulator-ubuntu	28m 27s
network-tests	16m 55s
op-pool-tests	19m 30s
no-eth1-simulator-ubuntu	26m 41s
release-tests-ubuntu	16m 36s
release-tests-windows	23m 18s
slasher-tests	5m 46s
state-transition-vectors-ubuntu	11m 5s
target-branch-check	1s
syncing-simulator-ubuntu	21m 12s
test-suite-success	5s
	5h 53m 47s

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