[FLINK-22120] Remove duplicate code in generated code for map get

[zoudan]

What is the purpose of the change

This pull request fixed a bug that there is duplicate code in generated code for map get, an it may lead to some exceptions such as 'Redefinition of local variable'.

Brief change log

remove the duplicate code in generated code for map get.

Verifying this change

This change added tests and can be verified as follows:

• org.apache.flink.table.planner.runtime.stream.sql.CalcITCase#testMapGet

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/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 / docs / JavaDocs / not documented)

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Automated Checks

Last check on commit f4c660a (Fri May 28 09:02:41 UTC 2021)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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CI report:

• f4c660a Azure: FAILURE

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[wuchong]

LGTM.

[wuchong]

The failed test case is not related to this PR. Will merge this.