[FLINK-13008]fix the findbugs warning in AggregationsFunctio.scala

[wangpeibin713]

What is the purpose of the change

• The goal is to fix the findbugs warning in AggregationsFunctio.scala
  https://issues.apache.org/jira/browse/FLINK-13008

Brief change log

• throw the unsupportedOperationException rather than return it

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: no
• 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?

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Automated Checks

Last check on commit d9fdee4 (Thu Oct 24 18:01:25 UTC 2019)

Warnings:

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

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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[wangpeibin713]

cc @zentol @rmetzger
could you review this trivial change

[flinkbot]

CI report:

• bea9f0b : FAILURE Build
• a31d440 : CANCELED Build
• d9fdee4 : SUCCESS Build

[wangpeibin713]

@hequn8128 @twalthr could you review this trivial change

[aljoscha]

Thanks! I merged this: ☕️