Fix JSON logging with null formatter keys

[xfocus3]

Summary

• Avoid checking excludedKeys with a null formatter key before delegating JSON generator calls.
• Reuse the same null-safe exclusion check in both JSON generator wrappers.
• Add regression coverage for null formatter keys and exception formatting when excluded keys are configured.

---

• Fixes: Formatting fails with a NullPointerException and the log record is dropped #54925