Core: Added {} and null support for default values within SchemaParser

[Verest]

Overview

SchemaParser does not support parsing schema's default values containing {} (structs) or explicit nulls (any supported data type). From the spec:

• When an optional field is added, the defaults may be null and should be explicitly set
• The default value of a struct can be either null or {}

Struct Change

This PR adds a new Literal for specifically EmptyStructLike to represent {} values. I reused the existing EmptyStructLike, but made it public which will expose to the class to consumers of iceberg-api and by extension iceberg-core.

I did not tether the Literal to the generic StructLike interface as:

• A non-empty StructLike Literal has no functional use at the moment.
• A non-empty StructLike Literal seemed riskier for potential side effects, given Literal is used outside of default values.
• Determining if a StructLike is empty appeared to require a recursive search for non-null values, which is unnecessary complexity.

I did not add conversions between String / EmptyStructLike Literals as that conversion is unrelated to this PR and probably undesired.

Null Change

This PR is leaving existing behavior around nulls default values for Schema -> JSON parsing. A small refactor was done to support null default values in metadata.json files when parsing JSON -> Schema.

Additional changes (if needed) around Schema -> JSON parsing can be done in another PR.

Asides

• Assumed boolean supportsDefaultValues() method of DataTestBase is N/A for TestSchemaParser as the underlying implementation defers to a writeAndValidate method. My changes are unrelated to writing the actual default data.
• Assuming the goal of {} is for a writer engine to be able to determine if a default struct value is null or Empty. I.e. using EmptyStructType as a flag for this scenario, so writers can react appropriately.
• Related thread on slack

[Verest]

@rdblue Here is a draft PR adding in null / {} supports for default values. Pinging you as requested.

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that’s incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.

[Verest]

Needs reviewed.

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that’s incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.

[github-actions]

This pull request has been closed due to lack of activity. This is not a judgement on the merit of the PR in any way. It is just a way of keeping the PR queue manageable. If you think that is incorrect, or the pull request requires review, you can revive the PR at any time.