[FLINK-33947][configuration] Fix bugs about the prefix attribute in the DelegatingConfiguration class.

[RocMarshal]

What is the purpose of the change

[FLINK-33947][configuration] Fix bugs about the prefix attribute in the DelegatingConfiguration class.

It was resulted from #23994 (comment)

• Check and confirm other potential bug points

• Fix the bugs about prefix key mapping when operating.

Brief change log

• Marked prefix as @Nonnull and adapted assignment about prefix
• Fixed bug about methods removeKey & removeConfig

Verifying this change

• DelegatingConfigurationTest#testRemoveKeyOrConfig

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

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

Documentation

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

[flinkbot]

CI report:

• c9405b8 Azure: SUCCESS

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[1996fanrui]

Thanks @RocMarshal for the fix!

LGTM.