Pipe: Fixed the NPE of OPC UA Sink in table model when there are null values & Deleted the extra folder of opc ua sink in table model

[Caideyipi]

Description

As the title said.

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious
  for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold
  for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

[Copilot]

Pull Request Overview

This PR addresses an NPE in the OPC UA Sink when encountering null values and removes an unnecessary folder level in the table model.

• Adjusts the length of the folder segments array to eliminate an extra folder.
• Adds a null check in the tablet value conversion to prevent an NPE.
• Skips null measurement values during OPC UA node creation.

Comments suppressed due to low confidence (3)

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/pipe/connector/protocol/opcua/OpcUaNameSpace.java:152

• Please confirm if the change from segments.length + 2 to segments.length + 1 correctly reflects the intended folder hierarchy adjustment without impacting downstream processing.

final String[] folderSegments = new String[segments.length + 1];

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/pipe/connector/protocol/opcua/OpcUaNameSpace.java:167

• [nitpick] Confirm that returning null in place of a tablet value is acceptable for OPC UA processing to avoid potential side effects downstream.

tablet.isNull(finalI, index) ? null : getTabletObjectValue4Opc(

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/pipe/connector/protocol/opcua/OpcUaNameSpace.java:233

• [nitpick] Ensure that skipping null measurement values will not cause misalignment in the sequence of measurements expected later in the process.

if (Objects.isNull(values.get(i))) {