Fixes crash in BookmarksApiUnittest.Update unit test.

The crash happens due to dereferencing an unintialized pointer.
This is fixed on master as part of
#3692, specifically in
054fd5a
but I don't think we should be uplifting all that just for a unit test
crash. So, instead just added a pointer null check.

browser/extensions/brave_extension_management.cc

@@ -44,7 +44,8 @@ void BraveExtensionManagement::RegisterBraveExtensions() {
 #if BUILDFLAG(ENABLE_TOR)
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
-  if (!command_line.HasSwitch(switches::kDisableTorClientUpdaterExtension))
+  if (g_brave_browser_process &&
+      !command_line.HasSwitch(switches::kDisableTorClientUpdaterExtension))
     g_brave_browser_process->tor_client_updater()->Register();
 #endif
 }