[Fix][#18160] Prevent NPE in SessionConnection when query response is null

[AdamsF-creator]

Description

Fixes #18160

Content1: Background & Problem

When executing queries via SessionConnection, if the RPC layer or network retry fails to obtain a valid response from the server, result.getResult() can be null. The client previously invoked execResp.getStatus() directly without verifying if execResp was null, leading to an unexpected NullPointerException (NPE) rather than a descriptive connection exception.

Content2: Defensive Null Check & Exception Handling

Added defensive null-checks for execResp / tsExecuteStatementResp across all relevant query methods (executeQueryStatement, executeRawDataQuery, executeLastDataQuery, executeLastDataQueryForOneDevice, executeLastDataQueryForOnePrefixPath, and executeAggregationQuery) in SessionConnection.java. When a null response is encountered, it now explicitly throws an IoTDBConnectionException("Cannot get statement response from server, response is null") to clearly indicate a communication failure.

Content3: Unit Test & Verification

Added a corresponding unit test testExecuteQueryStatementNullResponse in SessionConnectionTest.java by mocking IClientRPCService.Iface to return null. Verified that SessionConnection throws IoTDBConnectionException instead of crashing with an NPE.

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This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.

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

• org.apache.iotdb.session.SessionConnection
• org.apache.iotdb.session.SessionConnectionTest