Remove redundant nil checks in signer tests

[soniseth0]

simplify the signer/core signature tests by dropping the redundant signature == nil guard, rely on len(signature) != 65, which already covers nil slices and keeps the intent clearer

[gballet]

Hi, thanks a lot for your contribution! 🙏
At the moment, the go-ethereum team is overloaded with pull requests and in order to ensure we can give high quality review and attention to critical changes, we are not accepting refactoring-only PRs or minor upgrades.

We really appreciate the effort and encourage you to keep contributing, but for now we need to focus our limited review bandwidth on feature work, bug fixes, and protocol changes.

Thanks for understanding!