Fix chrome://inspect crash for optional USB serial_number.

Bug: 987002
Change-Id: I26cac0a7d9db6b47339eae5953ca11c99200c432
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716396
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680482}

chrome/browser/devtools/device/usb/usb_device_manager_helper.cc

@@ -64,7 +64,7 @@ void GetAndroidDeviceInfoList(
     std::vector<device::mojom::UsbDeviceInfoPtr> usb_devices) {
   std::vector<AndroidDeviceInfo> result;
   for (auto& device_info : usb_devices) {
-    if (device_info->serial_number->empty())
+    if (!device_info->serial_number || device_info->serial_number->empty())
       continue;
 
     auto interface_info = FindAndroidInterface(*device_info);