Fix browser_exe might be null, if not launching browser.

Author: juj

emrun.py

@@ -1803,7 +1803,7 @@ def run(cmd):
 
   # Pass the URL to open as the very last item on the command line, and use the -url xxx parameter
   # to open the url to work around https://bugzil.la/1996614.
-  if not options.android:
+  if browser_exe and not options.android:
     if 'firefox' in browser_exe:
       browser += ['-url', url]
     else: