[feat][client] Support null value messages

[Bhargavkonidena]

Fixes #489

Motivation

Currently the producer cannot send a null value message, which is used as a tombstone for a specific key on a compacted topic.
In addition, the consumer cannot differentiate empty values from null values, because all these values are presented as empty std::strings.

Modifications

1. Added MessageBuilder::setNullValue() method that sets the null_value field in MessageMetadata
2. Added Message::hasNullValue() method to check if a message has null value
3. Updated batch message serialization (Commands.cc) to preserve null_value in SingleMessageMetadata
4. Updated batch message deserialization (Message.cc) to copy null_value from SingleMessageMetadata
5. Added C API functions: pulsar_message_set_null_value() and pulsar_message_has_null_value()
6. Added unit tests and integration tests

Verifying this change

• Make sure that the change passes the CI checks.

Unit Tests (no broker required):

• MessageTest.testNullValueMessage - Basic null value API functionality

• MessageTest.testEmptyMessage - Verifies empty message is NOT a null value

• MessageTest.testEmptyStringNotNullValue - Verifies empty string "" is NOT the same as null value

• BatchMessageTest.testParseMessageBatchEntryWithNullValue - Verifies null value survives batch serialization/deserialization
  Integration Tests (require Pulsar broker):

• TableViewTest.testNullValueTombstone - Send tombstone, verify key removed from TableView

• TableViewTest.testNullValueVsEmptyString - Compare empty string vs null value behavior

• ReaderTest.testReadCompactedWithNullValue - Reader detects tombstones with hasNullValue()

• ReaderTest.testNullValueMessageProperties - Null value message preserves properties

Documentation

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[Copilot]

Pull request overview

Adds first-class “null value” (tombstone) support to the Pulsar C++ client so producers can emit tombstones on compacted topics and consumers can distinguish null vs empty payloads, including correct handling for batched messages and C bindings.

Changes:

• Introduces MessageBuilder::setNullValue() and Message::hasNullValue() (plus C API equivalents) to represent/inspect tombstone messages.
• Preserves null_value through batch message serialization/deserialization.
• Adds unit + integration tests covering null value behavior across Message, batch parsing, TableView, and Reader scenarios.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/TableViewTest.cc	Adds integration tests validating TableView tombstone behavior and empty-vs-null scenarios.
tests/ReaderTest.cc	Adds integration tests for reading compacted topics with tombstones and tombstone property preservation.
tests/MessageTest.cc	Adds unit tests for basic null value vs empty message semantics.
tests/BatchMessageTest.cc	Adds unit test ensuring null_value survives batch serialize/deserialize.
lib/c/c_Message.cc	Exposes C API wrappers to set/check null value.
lib/MessageBuilder.cc	Implements MessageBuilder::setNullValue().
lib/Message.cc	Implements Message::hasNullValue() and copies null_value from SingleMessageMetadata for batch entries.
lib/Commands.cc	Serializes null_value into SingleMessageMetadata for batch payloads.
include/pulsar/c/message.h	Declares/document C API for null value.
include/pulsar/MessageBuilder.h	Declares/documents setNullValue().
include/pulsar/Message.h	Declares/documents hasNullValue().

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[BewareMyPower]

Overall LGTM, please fix the code format and address comments from copilot

code format check failed