Update docs so that responseText returns empty String on failure.

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 399 matching lines @@
           "/**",
           "   * The data received as a reponse from the request.",
           "   *",
           "   * The data could be in the",
           "   * form of a [String], [ByteBuffer], [Document], [Blob], or json (also a",
           "   * [String]). `null` indicates request failure.",
           "   */"
         ],
         "responseText": [
           "/**",
-          "   * The response in string form or `null on failure.",
+          "   * The response in String form or empty String on failure.",
           "   */"
         ],
         "responseType": [
           "/**",
           "   * [String] telling the server the desired response format.",
           "   *",
           "   * Default is `String`.",
           "   * Other options are one of 'arraybuffer', 'blob', 'document', 'json',",
           "   * 'text'. Some newer browsers will throw NS_ERROR_DOM_INVALID_ACCESS_ERR if",
           "   * `responseType` is set while performing a synchronous request.",
@@ skipping 111 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }