Core: Avoid NPE when getting updateEvent in FastAppend

[wForget]

Similar to #2552, the NPE may also occur in the FastAppend#updateEvent method, and we should avoid it like that.

[wForget]

BTW, why use ops.refresh().snapshot() instead of ops.current().snapshot() in MergingSnapshotProducer#updateEvent?

iceberg/core/src/main/java/org/apache/iceberg/MergingSnapshotProducer.java

Line 870 in 2b18975

Snapshot justSaved = ops.refresh().snapshot(snapshotId);

[github-actions]

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