[FLINK-14642] [types] Add support for copying null values to the TupleSeriali…

[jiasheng55]

…zer and CaseClassSerializer

What is the purpose of the change

This PR makes the copy method of the TupleSerializer and CaseClassSerializer to handle NULL values properly, so elements of these types can be passed between chained operators.
Without this PR, the following code may throw NPE in case of map function returns null, which may confuse the user.

stream.map(xxx).filter(_ != null).xxx //the return type of the map function is Tuple and it may return null

Brief change log

• return null if the element itself is null.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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: (yes)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, 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? (not applicable)

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Automated Checks

Last check on commit 982f9e3 (Wed Dec 04 15:08:36 UTC 2019)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!
• This pull request references an unassigned Jira ticket. According to the code contribution guide, tickets need to be assigned before starting with the implementation work.

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Review Progress

• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• 982f9e3 : SUCCESS Build

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[jiasheng55]

@rmetzger Could you help take a look at this PR:)