fix(textarea): 修复 slot 嵌套使用 textarea 时的输入失焦问题 (#3084)

Author: eiinu

src/packages/__VUE/textarea/textarea.taro.vue

@@ -11,7 +11,7 @@
       :show-count="false"
       :maxlength="maxLength ? maxLength : -1"
       :placeholder="placeholder || translate('placeholder')"
-      :auto-focus="autofocus"
+      :auto-focus="autofocus ? true : undefined"
       @input="change"
       @blur="blur"
       @focus="focus"