Fix iceberg $changelog read failing on single snapshot

[npsparrow]

Description

Introduced a condition when reading an iceberg $changelog table so that if only one snapshot exists, it returns an empty table.

Motivation and Context

Previously, attempting to read from an iceberg changelog would fail.
This fixes this issue.

Impact

Allows connector to query Iceberg changelog without failure when a single snapshot exists.

Test Plan

Introduced a unit test testGetChangelogWithSingleSnapshot in TestIcebergSplitManager.java.

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Release Notes

== NO RELEASE NOTE ==

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: npsparrow / name: Nikhil Panickssery (e6ffbcc)