fix bug in getting-started - date is null when clear the DatePick (#10111)

docs/react/getting-started.en-US.md

@@ -79,7 +79,7 @@ class App extends React.Component {
     };
   }
   handleChange(date) {
-    message.info('Selected Date: ' + date.toString());
+    message.info('Selected Date: ' + (date ? date.toString() : ''));
     this.setState({ date });
   }
   render() {
@@ -87,7 +87,7 @@ class App extends React.Component {
       <LocaleProvider locale={frFR}>
         <div style={{ width: 400, margin: '100px auto' }}>
           <DatePicker onChange={value => this.handleChange(value)} />
-          <div style={{ marginTop: 20 }}>Date: {this.state.date.toString()}</div>
+          <div style={{ marginTop: 20 }}>Date: {this.state.date && this.state.date.toString()}</div>
         </div>
       </LocaleProvider>
     );

docs/react/getting-started.zh-CN.md

@@ -81,7 +81,7 @@ class App extends React.Component {
     };
   }
   handleChange(date) {
-    message.info('您选择的日期是: ' + date.toString());
+    message.info('您选择的日期是: ' + (date ? date.toString() : ''));
     this.setState({ date });
   }
   render() {
@@ -89,7 +89,7 @@ class App extends React.Component {
       <LocaleProvider locale={zhCN}>
         <div style={{ width: 400, margin: '100px auto' }}>
           <DatePicker onChange={value => this.handleChange(value)} />
-          <div style={{ marginTop: 20 }}>当前日期：{this.state.date.toString()}</div>
+          <div style={{ marginTop: 20 }}>当前日期：{this.state.date && this.state.date.toString()}</div>
         </div>
       </LocaleProvider>
     );