[WIP][FLINK-34723][parquet] Restrict parquet map keys to be not null

[xccui]

What is the purpose of the change

We hit the following exception when reading a parquet file generated by Flink.

Map keys must be annotated as required.

Brief change log

Restricted the map key type to be not null in ParquetSchemaConverter.

Verifying this change

• Modified the existing tests to use not null key types.
• Added a new test testNullableMapKeyNotAllowed.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (doc updated)

[flinkbot]

CI report:

• 43c51d3 Azure: FAILURE

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