修复当FileRefreshableDataSource文件为空时读取异常

[genjigjj]

Describe what this PR does / why we need it

在使用FileRefreshableDataSource作为规则数据源时，发现如果初始化空规则文件会导致StringIndexOutOfBoundsException异常

Does this pull request fix one issue?

Describe how you did it

判断规则文件是否有数据

Describe how to verify it

加载空的规则文件看是否有异常打印

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