这与使用空字符串不同，空字符串表示属性存在但是没有值。
This is different from using the empty string, which indicates that the property exists but has no value.
标题(title)，简短的、人类可读的记录主体行；允许使用空字符串(用空的标题元素表示，如)。
A title, which expresses a short, human readable subject line for the entry, and can be a blank string (represented by an empty title element, such as).
scope：搜索范围；空字符串表示搜索所有可用的索引页。
Scope: The scope to search; an empty string means search all available indexes.
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