Backport PR #6105: only set allow_origin_pat if defined

fixes the default behavior to be as intended (require Origin == Host)

IPython/html/notebookapp.py

@@ -631,7 +631,8 b' class NotebookApp(BaseIPythonApplication):'
     def init_webapp(self):
         """initialize tornado webapp and httpserver"""
         self.webapp_settings['allow_origin'] = self.allow_origin
-        self.webapp_settings['allow_origin_pat'] = re.compile(self.allow_origin_pat)
+        if self.allow_origin_pat:
+            self.webapp_settings['allow_origin_pat'] = re.compile(self.allow_origin_pat)
         self.webapp_settings['allow_credentials'] = self.allow_credentials
 
         self.web_app = NotebookWebApplication(