fix: getPoolAttestationsV2 should return an empty array when no attestations are available

[markolazic01]

Motivation

Initially this endpoint returned 500 when there are no attestations for a specified slot, instead it should return an empty list.

Description

Simple single-line change replacing 500 with an empty array return - as described in the issue itself.

Closes #9669

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[nflaig]

did you double check against the spec? I am not sure this is correct, I feel like the issue that lodekeeper reported is due to the fact that we cannot serve attestations from pool which we should in that case. for some context, vouch queries to pool to determine whether the main instance is active or not, so the hot standby can be activated. I would need to dig more into why we returned 500 in the first place so the pool should have an attestation since the vouch instance itself submitted it. anyhow this doesn't really fix the root cause

[markolazic01]

got it, I added this only as I started inspecting the topic, returning 500 seemed wrong so I went with it (seems spec aligns with using 500 only for when internal errors occur).
there is a separate issue created which focuses on the root cause of this #9677.