Fix tooltip overlay warning, close #6897

components/tooltip/demo/basic.md

@@ -17,7 +17,7 @@ The simplest usage.
 import { Tooltip } from 'antd';
 
 ReactDOM.render(
-  <Tooltip title="prompt text">
+  <Tooltip title={undefined}>
     <span>Tooltip will show when mouse enter.</span>
   </Tooltip>
 , mountNode);

components/tooltip/index.tsx

@@ -206,7 +206,7 @@ export default class Tooltip extends React.Component<TooltipProps, any> {
         getTooltipContainer={getPopupContainer || getTooltipContainer}
         ref="tooltip"
         builtinPlacements={this.getPlacements()}
-        overlay={overlay || title}
+        overlay={overlay || title || ''}
         visible={visible}
         onVisibleChange={this.onVisibleChange}
         onPopupAlign={this.onPopupAlign}