spec: clarity `AssertRefSnapshotId`'s `snapshot-id` field as required

[kevinjqliu]

Apache Iceberg version

None

Query engine

None

Please describe the bug 🐞

AssertRefSnapshotId's snapshot-id field is required even though it can be null or missing. Call out this specific requirement as some serializers might omit missing/null fields by default.

this is the only case of required but null/missing in the REST spec

We found this when working with pyiceberg apache/iceberg-python#2343

Willingness to contribute

• I can contribute a fix for this bug independently
• I would be willing to contribute a fix for this bug with guidance from the Iceberg community
• I cannot contribute a fix for this bug at this time