DevTools: do not assign empty content to CSS UISourceCode in case of protocol error

third_party/WebKit/Source/devtools/front_end/common/StaticContentProvider.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 /**
  * @implements {Common.ContentProvider}
  * @unrestricted
  */
 Common.StaticContentProvider = class {
   /**
    * @param {string} contentURL
    * @param {!Common.ResourceType} contentType
-   * @param {function():!Promise<string>} lazyContent
+   * @param {function():!Promise<?string>} lazyContent
    */
   constructor(contentURL, contentType, lazyContent) {
     this._contentURL = contentURL;
     this._contentType = contentType;
     this._lazyContent = lazyContent;
   }
 
   /**
    * @param {string} contentURL
    * @param {!Common.ResourceType} contentType
@@ skipping 31 matching lines @@
 
   /**
    * @override
    * @param {string} query
    * @param {boolean} caseSensitive
    * @param {boolean} isRegex
    * @param {function(!Array.<!Common.ContentProvider.SearchMatch>)} callback
    */
   searchInContent(query, caseSensitive, isRegex, callback) {
     /**
-     * @param {string} content
+     * @param {?string} content
      */
     function performSearch(content) {
+      if (!content) {
+        callback(/** @type {!Array<!Common.ContentProvider.SearchMatch>} */ ([]));
+        return;
+      }
       callback(Common.ContentProvider.performSearchInContent(content, query, caseSensitive, isRegex));
     }
 
     this._lazyContent().then(performSearch);
   }
 };