修复 TreeSelect 单选不可以清除选项的问题

src/components/ResultBox.tsx

@@ -173,7 +173,9 @@ export class ResultBox extends React.Component<ResultBoxProps> {
 
         {children}
 
-        {clearable && !disabled && Array.isArray(result) && result.length ? (
+        {clearable &&
+        !disabled &&
+        (Array.isArray(result) ? result.length : result) ? (
           <a
             // data-tooltip="清空"
             // data-position="bottom"

src/renderers/Form/TreeSelect.tsx

@@ -327,11 +327,17 @@ export default class TreeSelectControl extends React.Component<
       extractValue,
       delimiter,
       valueField,
-      onChange
+      onChange,
+      multiple
     } = this.props;
 
     let newValue: any = Array.isArray(value) ? value.concat() : [];
 
+    if (!multiple && !newValue.length) {
+      onChange('');
+      return;
+    }
+
     if (joinValues || extractValue) {
       newValue = value.map(item => item[valueField || 'value']);
     }