[FLINK-39909][state] Fix heap state backend savepoint NPE with null map state values

[mukul-8]

What is the purpose of the change

Found while investigating FLINK-38144. HeapKeyValueStateIterator unconditionally calls userValueSerializer.serialize(userValue, valueOut) after writing the null flag, causing NPE during canonical savepoint when MapState contains null values with null-unsafe serializers (IntSerializer, BooleanSerializer, LongSerializer, etc.).

Same asymmetric pattern fixed for RocksDB/ForSt in #26831 (FLINK-38137). Only affects canonical savepoints — heap checkpoints use MapSerializer.serialize() which handles nulls internally

Brief change log

• Skip userValueSerializer.serialize() when value is null in HeapKeyValueStateIterator, matching the deserialization side which already checks the null flag
• Add testMapStateWithNullUnsafeSerializerCheckpointingAndRestore to MapStateNullValueCheckpointingITCase using IntSerializer (null-unsafe) to cover this code path

Verifying this change

• Added integration test testMapStateWithNullUnsafeSerializerCheckpointingAndRestore — runs across all backends (hashmap, rocksdb, forst) and snapshot types (full checkpoint, incremental checkpoint, canonical savepoint, native savepoint)
• Manually verified with a long-running job using hashmap backend + null map state values + savepoint/restore

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, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  kiro-cli

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CI report:

• 7bd95c0 Azure: SUCCESS

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