Fix Beacon API host validation in validator service

[maradini77]

## Bug Fix

**What does this PR do? Why is it needed?**

Fixes a logic error in Beacon API host validation where empty or whitespace-only endpoints would pass validation and cause the validator service to start with an invalid configuration.

**Problem**

The original code used `strings.Split()` which always returns at least one element (even if empty), making the `len(hosts) == 0` check ineffective. When `BeaconApiEndpoint` was empty or contained only commas/whitespace, the service would proceed to create a REST client with an empty base URL, leading to runtime failures during HTTP requests.

**Solution**

- Filter out empty strings from the split result before validation
- Ensure the service fails fast with a clear error message when no valid hosts are provided
- Prevent invalid REST client initialization

**Changes**

- Added filtering loop to remove empty host entries
- Improved validation to catch empty/malformed endpoint configurations early

[james-prysm]

please add a unit test and changelog