[FLINK-32665][format] Support reading null value for csv format

[FangYongs]

What is the purpose of the change

Currently we can create table with nullable column for csv format table, but it will throw exception if there's null value in the record for these columns. This PR aims to support reading null value for csv format.

Brief change log

• Return null value for int/long/double/boolean when the text is empty

Verifying this change

This change added tests and can be verified as follows:

• Added CsvFormatFactoryTest.testDeserializeNullValue to read null value

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

• Dependencies (does it add or upgrade a dependency): (yes / no) no
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no) no
• The serializers: (yes / no / don't know) no
• The runtime per-record code paths (performance sensitive): (yes / no / don't know) no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know) no
• The S3 file system connector: (yes / no / don't know) no

Documentation

• Does this pull request introduce a new feature? (yes / no) no
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• da9ceca Azure: SUCCESS

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[FangYongs]

@libenchao Please help to review this PR when you're free, thanks

[libenchao]

createNullableConverter should have already handled nulls (similar to json format), however, for csv, this does not work because blank fields is represented as empty text node (empty string). I'm not sure whether there is any configuration for jackson csv mapper to represent blank fields to a null node?

[libenchao]

There is already a config in csv format about null literal null-literal, I guess setting it to empty string would solve the problem?

[FangYongs]

I try the option null-literal and it works. But I think there're some improvement points about csv format here. Currently when I create a csv format table with nullable fields such as table ( a int, b int). There are no additional options for the table and I can successfully insert null data to the field a and b. When I try to select results from the table, it throw exception that empty string('') cannot be parsed as int value. I think there may be two solutions for this, and we can pick one:

1. Throw an exception and tell users to configure null-literal for their csv table if they try to insert a null value.
2. Give the option null-literal a default value such as empty string('')

@libenchao What do you think of it?

[MartijnVisser]

2. Give the option null-literal a default value such as empty string('')

That would be a breaking change for existing users, which I don't think we should do here.

[libenchao]

I also have the concern of the breaking change. Then we can choose 1 as a temporary solution.

But eventually, it would be good to improve the out-of-box experience, and go for 2 finally, maybe in Flink 2.0?

[MartijnVisser]

and go for 2 finally, maybe in Flink 2.0?

Do propose it for Flink 2.0 indeed!