fix: add config payload non-empty check in setConfig

[92ikonostasov]

What this PR does

Adds a require check in setConfig(...) to ensure that the _config payload is not empty. This prevents accidental setting of empty configurations and improves overall contract robustness.

Why it's useful

• Protects against misconfigurations or unintended empty payloads
• Improves input validation in a critical configuration function
• Aligned with best practices used throughout the LayerZero contracts

Implementation

Added:

require(_config.length > 0, "LayerZero: config payload cannot be empty");