gh-38005: Fix Help menu breakdown in external jupyterlab case

    
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#37878 introduced a local documentation server for jupyterlab. But it
missed a case for which the Help menu of jupyterlab may break down.

In the setup explained in
https://doc.sagemath.org/html/en/installation/launching.html#setting-up-
sagemath-as-a-jupyter-kernel-in-an-existing-jupyter-notebook-or-
jupyterlab-installation, external jupyterlab is launched and connected
to sage kernel. In this case, the local doc server does not run, but the
Help menu of the jupyterlab still expects a local doc server, and thus
the Help menu does not work well.

In this PR, local doc server is detected by testing the port number, and
the Help menu uses the online official documentation if local doc server
is not running.

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preview.

### ⌛ Dependencies

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URL: #38005
Reported by: Kwankyu Lee
Reviewer(s):

src/sage/repl/ipython_kernel/kernel.py

@@ -108,14 +108,13 @@ def help_links(self):
         # src/bin/sage-notebook.
 
         from sage.env import SAGE_DOC_SERVER_URL
+        from sage.env import SAGE_DOC_LOCAL_PORT as port
         from sage.features.sagemath import sagemath_doc_html
 
         if SAGE_DOC_SERVER_URL:
             def doc_url(path):
                 return f'{SAGE_DOC_SERVER_URL}/{path}'
-        elif sagemath_doc_html().is_present():
-            from sage.env import SAGE_DOC_LOCAL_PORT as port
-
+        elif sagemath_doc_html().is_present() and int(port):
             def doc_url(path):
                 return f'http://127.0.0.1:{port}/{path}'
         else: