fix(excel-import): guard excelReader.close() against null in finally

[Aias00]

Related issue

Closes #2033

Summary

In DbExcelTableServiceImpl, ExcelReader excelReader = null is assigned via EasyExcel.read(...).build() inside a try. If build() (or read) throws — e.g. a corrupt/IO-error file — excelReader is still null, and the finally { excelReader.close(); } block throws a NullPointerException that masks the real root cause. Added a null-check before close() at both import sites.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results: mvn -B -q -f chat2db-community-server/pom.xml -pl chat2db-community-domain/chat2db-community-domain-core -am -Dmaven.test.skip=true compile -> BUILD SUCCESS.
• Manual verification: When build() throws, excelReader remains null, the guarded finally skips close(), and the original exception propagates via the catch (Exception e) { throw new RuntimeException(e); }.
• UI evidence: N/A

Risk and compatibility

• Public API or stored data: N/A.
• Database or driver compatibility: N/A.
• Network, privacy, or security: N/A.
• Community / Local / Pro boundary: N/A.
• Backward compatibility: Only changes the failure path from "NPE masks real error" to "real error propagates"; success path unchanged.

Reviewer map

• Start here: DbExcelTableServiceImpl.java finally blocks at the two import methods — excelReader.close(); -> if (excelReader != null) { excelReader.close(); }.
• Failure condition: A NullPointerException from excelReader.close() still masks the real parse/IO error.
• Rollback or disable path: Revert this single commit.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: The fix, verification, and PR description were produced with Claude Code assistance.

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[openai0229]

Reviewed both Excel reader cleanup paths. The null guards preserve the original initialization failure instead of masking it with a finally-block NPE; no blocking issue found.