fix: validate internal tsfile paths for files only

[luoluoyuyu]

Description

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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

[Caideyipi]

[P1] Table Model 的 LOAD 路径没有经过这里的空路径校验。

关系型解析器 AstBuilder.visitLoadTsFileStatement() 会直接构造 LoadTsFile；该类的构造函数只检查 null，随后执行 processTsFile(new File(filePath), ...)。因此关系型语法 LOAD '' 仍会把空字符串解析成进程当前工作目录，可能递归扫描并加载其中的 .tsfile（至少也不会返回本 PR 新增的“LOAD TSFILE path cannot be empty”错误）。RelationalSql.g4 的 STRING 规则允许空字符串。

请把校验抽成字符串级 helper，在 LoadTsFileStatement 和 LoadTsFile 两个入口复用，并补充 Tree/Table 两种 dialect 的 LOAD '' 回归测试。