[Bug] Committing only null partition values fails with NPE when Iceberg compatibility is enabled

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Paimon version
master @ 158d83c (2.1-SNAPSHOT)

Compute Engine
Engine-agnostic (core)

Minimal reproduce step

1. Create pt VARCHAR(10), v INT partitioned by pt, bucket -1, with metadata.iceberg.storage = table-location.
2. Commit a row with pt = 'a'.
3. Commit a batch in which every row has pt = NULL — this fails:

java.lang.NullPointerException
  at IcebergConversions.toByteBuffer(IcebergConversions.java:83)
  at IcebergManifestFile$IcebergManifestEntryWriter.result(IcebergManifestFile.java:273)
  at IcebergCommitCallback.call(IcebergCommitCallback.java:256)

What doesn't meet your expectations?
The commit should succeed. Paimon supports null partition values, and the Iceberg spec declares field_summary.lower_bound (510) and upper_bound (511) optional — "or null if all values are null or NaN". IcebergPartitionSummary.schema() already declares both nullable.

Anything else?
When every entry of a manifest holds a null partition value, the collected SimpleColStats min/max stay null and reach toByteBuffer unguarded. The sibling IcebergDataFileMeta.create() has omitted unknown column bounds since #4090; the partition summary path never got the equivalent guard. Reading is affected too: IcebergCommitCallback.createWithDeleteManifestFileMetas hits the same null in toPaimonObject on the next non add-only commit.

Are you willing to submit a PR?

• I'm willing to submit a PR!