Swift_集合的isEmpty属性

【使用】：一个Bool值，指示集合是否为空。
值得注意的是：字符串是字符的集合，也遵循了RandomAccessCollection。

/// When you need to check whether your collection is empty, use the `isEmpty` property instead of checking that the `count` property is equal to zero. For collections that don't conform to `RandomAccessCollection`, accessing the `count` property iterates through the elements of the collection.
// ：当你需要检查集合是否为空时，使用' isEmpty '属性代替检查，而不是检查' count '属性等于零。对于不符合“RandomAccessCollection”的集合，访问“count”属性将遍历集合的元素。
///
///     let horseName = "Silver"
///     if horseName.isEmpty {
///         print("I've been through the desert on a horse with no name.")
///     } else {
///         print("Hi ho, \(horseName)!")
///     }
///     // Prints "Hi ho, Silver!")
///
    @inlinable public var isEmpty: Bool { get }