Fix nullness in some of Beam SQL

[kennknowles]

Just fixing stuff up as I re-learn this corner of the codebase.

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Summary of Changes

Hello @kennknowles, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the nullness safety and clarity within the Beam SQL extension. By systematically removing broad nullness suppression annotations, introducing explicit null checks at critical points, and applying @Nullable type annotations, the changes aim to make the codebase more robust against null pointer exceptions and easier to reason about for developers and static analysis tools. The modifications touch core components related to schema management, table resolution, and UDF creation.

Highlights

• Nullness Annotations Removal: Removed numerous @SuppressWarnings({"keyfor", "nullness"}) annotations across several files, indicating a move towards stricter nullness checking and improved code quality within the Beam SQL extension.
• Explicit Null Checks: Introduced org.apache.beam.sdk.util.Preconditions.checkArgumentNotNull and checkStateNotNull in various methods to explicitly validate non-null arguments and states, preventing potential NullPointerExceptions at runtime.
• Nullable Type Annotations: Added @Nullable annotations to method parameters and return types in key interfaces and classes (Schema, TableProvider, PipelineOptions, etc.) to clearly define nullability contracts, aiding static analysis and developer understanding.
• Refactored Schema and Table Resolution Logic: Updated logic in BeamCalciteSchema.getSubSchema, JdbcConnection.initialize, JdbcConnection.getCurrentBeamSchema, JdbcConnection.getCurrentSchemaPlus, and TableResolutionUtils to handle null sub-schemas and current schema names more robustly with explicit checks.
• UDF Creation Robustness: Modified UdfImpl.create to throw a RuntimeException if a specified method for UDF creation is not found, rather than silently returning null, ensuring clearer error reporting and preventing unexpected behavior.

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[kennknowles]

This (and other equivalent places in this file) are a semantics change. instead of returning null and letting things NPE later on, I fail here. If the method isn't found, I doubt there's anything to be done but crash out. I'll see if any CI/CD fails.

[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[kennknowles]

R: @ahmedabu98

[kennknowles]

Turns out we did have some unusual code that was in the style "propagate null as far as possible" which would get NPEs or other things showing up far from where something actually had a reason for returning null.

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control. If you'd like to restart, comment assign set of reviewers

[ahmedabu98]

LGTM

Can we run SQL postcommits just in case?