HarmonyOS 如何判定一个String字符串，为空或者为null

@Entry
@Component
struct Index {
  @State message: string|null = null;
  build() {
    Row() {
      Column() {
        if (this.message === null || this.message === ""){
          Text("ddd")
            .fontSize(50)
            .fontWeight(FontWeight.Bold)
        }else {
          Text("ewrwerwer")
            .fontSize(50)
            .fontWeight(FontWeight.Bold)
        }
      }
      .width('100%')
    }
    .height('100%')
  }
}
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