Core: Handle optional fields

[Fokko]

In the Java code, we expect:

• current-snapshot-id
• properties
• snapshots

to be there, but they are actually optional. Related #8049

Java still writes -1 for the current-snapshot-id when there is no snapshot, but I think it is important to fix the readers first.

[zinking]

LGTM

on the other side, there doesn't seem to be parsing exceptions from my experience, is that because the writer essentially always write out the optional field. in this sense, what is changed ? or it's just to stick more strictly with the spec?

[nastra]

LGTM with one nit

[Fokko]

on the other side, there doesn't seem to be parsing exceptions from my experience, is that because the writer essentially always write out the optional field. in this sense, what is changed ? or it's just to stick more strictly with the spec?

@zinking Thanks for the review. Let me elaborate. It is sticking to the spec on the read side. In the spec, the current-snapshot-id, properties, and snapshots fields can be omitted, but the parser doesn't allow this.

The Java code still writes a -1 when writing a Metadata JSON:

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

Lines 216 to 218 in 531bf9d

	generator.writeNumberField(
	CURRENT_SNAPSHOT_ID,
	metadata.currentSnapshot() != null ? metadata.currentSnapshot().snapshotId() : -1);

But I don't want to change that today, since other readers might expect a -1 instead of omitting the field (I know Athena does). I think aking sure that the read-side adheres to the spec is a good first step.

[Fokko]

Thanks @zinking @nastra for the review, and @szehon-ho for adding the milestone. Let me create the backport right away