Handle blank thumbnails in TabStripUIHandler

For security reasons, the thumbnail system must be able to clear the
thumbnail for a tab and notify subscribers. A recent CL implemented
this behavior but resulted in crashes in
TabStripUIHandler::HandleThumbnailUpdate

This CL fixes the crashes due to the null pointer access and handles
blank thumbnails by sending an empty URI to the WebUI.

Fixed: 1167502
Change-Id: I679500882b671b799f3a3c00b55ac20d8e1fb561
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638635
Auto-Submit: Collin Baker <collinbaker@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845280}

chrome/browser/ui/webui/tab_strip/tab_strip_ui_handler.cc

@@ -862,9 +862,11 @@ void TabStripUIHandler::HandleReportTabCreationDuration(
 void TabStripUIHandler::HandleThumbnailUpdate(
     content::WebContents* tab,
     ThumbnailTracker::CompressedThumbnailData image) {
-  // Send base-64 encoded image to JS side.
-  std::string data_uri =
-      webui::MakeDataURIForImage(base::make_span(image->data), "jpeg");
+  // Send base-64 encoded image to JS side. If |image| is blank (i.e.
+  // there is no data), send a blank URI.
+  std::string data_uri;
+  if (image)
+    data_uri = webui::MakeDataURIForImage(base::make_span(image->data), "jpeg");
 
   const int tab_id = extensions::ExtensionTabUtil::GetTabId(tab);
   FireWebUIListener("tab-thumbnail-updated", base::Value(tab_id),