Reconsider default fee recipient behavior

### Problem description

As noted in [tweet](https://twitter.com/peter_szilagyi/status/1767595768934719993) that there are a lot of produced blocks with [0x0000000000000000000000000000000000000000](https://etherscan.io/address/0x0000000000000000000000000000000000000000#mine) as fee recipient.

In Lodestar, this is the default fee recipient if a user doesn't explicitly set it via
- `--suggestedFeeRecipient` flag
- per pubkey via `--proposerSettingsFile`
- or through keymanager api

We could consider a more strict behavior, for example, requiring to [set a fee recipient at startup](https://twitter.com/nanexcool/status/1767620555820200235) for each validator pubkey but this might not be desired for all users as some might primarily use the keymanager API to configure their validators.

### Solution description

Better default behavior, for example, default to withdrawal address if no fee recipient is set, and if there is no withdrawal address, only then default to zero address.

### Additional context

_No response_