fix(docs): support optimized cx_Freeze packaging

[slook]

• Fixed: Avoid TypeErrors when using cx_Freeze to compile a Windows or Darwin binary containing a bundled TTk library

cx_Freeze can strip all docstrings (equivalent to Python's -OO flag) to optimize the executable file size, making __doc__ return None. The library then fails the testrun after build-time when it tries to concatenate None to a string.

Example CI packaging GH actions runner output (similar exception was on both my Windows and macOS builds):

Run python3 build-aux/windows/testrun.py
  python3 build-aux/windows/testrun.py
  shell: C:\Windows\system32\cmd.EXE /D /E:ON /V:OFF /S /C "CALL "{0}""
  env:
    ARCH: x86_64
    MSYSTEM: CLANG64
Using TTk executable: D:/a/nicotine-plus/nicotine-plus/build-aux/windows/build/package/Nicotine+/lib/pynicotine/external/pyTermTk
Windows
Traceback (most recent call last):
  File "D:/a/_temp/msys64/clang64/lib/python3.14/site-packages/freeze_core/initscripts/__startup__.py", line 137, in run
    module_init.run(f"__main__{name}")
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "D:/a/_temp/msys64/clang64/lib/python3.14/site-packages/freeze_core/initscripts/console.py", line 27, in run
    exec(code, main_globals)
    ~~~~^^^^^^^^^^^^^^^^^^^^
  File "D:/a/nicotine-plus/nicotine-plus/nicotine", line 30, in <module>
    sys.exit(load_module())
             ~~~~~~~~~~~^^
  File "D:/a/nicotine-plus/nicotine-plus/nicotine", line 20, in load_module
    return pynicotine.run()
           ~~~~~~~~~~~~~~^^
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/__init__.py", line 224, in run
    exit_code = application.run(ci_mode, isolated_mode)
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/ttktui/__init__.py", line 29, in run
    error = check_ttk_version()
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/ttktui/__init__.py", line 11, in check_ttk_version
    from TermTk import __version__ as TTk_version
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/__init__.py", line 25, in <module>
    from .TTkTheme       import *
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkTheme/__init__.py", line 1, in <module>
    from .theme import *
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkTheme/theme.py", line 25, in <module>
    from TermTk.TTkCore.color import TTkColor
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/__init__.py", line 8, in <module>
    from .ttk      import *
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkCore/ttk.py", line 47, in <module>
    from TermTk.TTkWidgets.about import TTkAbout
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkWidgets/__init__.py", line 16, in <module>
    from .combobox        import *
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkWidgets/combobox.py", line 38, in <module>
    from TermTk.TTkWidgets.list_ import TTkList
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkWidgets/list_.py", line 32, in <module>
    class TTkList(TTkAbstractScrollArea):
    ...<321 lines>...
            return self._listView.setCurrentItem(item=item)
  File "D:/a/nicotine-plus/nicotine-plus/pynicotine/external/pyTermTk/TermTk/TTkWidgets/list_.py", line 33, in TTkList
    __doc__ = '''
              ~~~
    :py:class:`TTkList` is a container widget which place :py:class:`TTkListWidget` in a scrolling area with on-demand scroll bars.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    
    ''' + TTkListWidget.__doc__
    ~~~~^~~~~~~~~~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "NoneType") to str