Fix when clear search text, all text split by blank (#3072)

in search-highlighting example, when search text is empty, no need to split text and addAnnotation

examples/search-highlighting/index.js

@@ -185,6 +185,7 @@ class SearchHighlighting extends React.Component {
         }
       })
 
+      if (string) {
         for (const [node, path] of document.texts()) {
           const { key, text } = node
           const parts = text.split(string)
@@ -203,6 +204,7 @@ class SearchHighlighting extends React.Component {
             offset = offset + part.length + string.length
           })
         }
+      }
     })
   }
 }