mkdocs-material: Optimize plugin fails with "No such file or directory" in .cache directory

Context

I recently tried adding the optimize plugin to work with a variety of PNGs I use on https://distant.dev/. It appears that I am hitting the issue described in #5098, so taking a stab at reporting it.

I’m using commit fa5bd3f45024328d973977db80cccd5b1df89228 of Insiders.

Bug description

Running mkdocs build or mkdocs serve with the optimize plugin configured within mkdocs.yml results in a FileNotFoundError.

❯ mkdocs build
INFO     -  [macros] - Macros arguments: {'module_name': 'macros', 'modules': [], 'render_by_default': True, 'include_dir': '',
            'include_yaml': [], 'j2_block_start_string': '', 'j2_block_end_string': '', 'j2_variable_start_string': '', 'j2_variable_end_string': '',
            'on_undefined': 'strict', 'on_error_fail': True, 'verbose': False}
INFO     -  [macros] - Found local Python module 'macros' in: /Users/senkwich/projects/distant.dev
INFO     -  [macros] - Found external Python module 'macros' in: /Users/senkwich/projects/distant.dev
INFO     -  [macros] - Extra filters (module): ['pretty']
INFO     -  Cleaning site directory
INFO     -  Building documentation to directory: /Users/senkwich/projects/distant.dev/site

  Optimizations:
Traceback (most recent call last):
  File "/Users/senkwich/Library/Python/3.9/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/mkdocs/__main__.py", line 250, in build_command
    build.build(cfg, dirty=not clean)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/mkdocs/commands/build.py", line 332, in build
    config.plugins.run_event('post_build', config=config)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/mkdocs/plugins.py", line 522, in run_event
    result = method(**kwargs)
  File "/Users/senkwich/Library/Python/3.9/lib/python/site-packages/material/plugins/optimize/plugin.py", line 113, in on_post_build
    size_opt += os.path.getsize(target)
  File "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/genericpath.py", line 50, in getsize
    return os.stat(filename).st_size
FileNotFoundError: [Errno 2] No such file or directory: '.cache/plugin/optimize/assets/favicon.png'

Related links

• Optimize Plugin

Reproduction

example.zip

I’ve removed everything but a blank index.md page as it appears unrelated to which images are actual referenced. Additionally, I cleared out the majority of images and other assets alongside all css and javascript.

mkdocs.yml

nav:
  - 'index.md'

theme:
  name: material
  favicon: assets/favicon.png
  logo: assets/logo.svg

plugins:
  - optimize

Steps to reproduce

1. Create a couple of PNGs in the assets directory
   • For me, a minimal layout is

     assets/
         favicon.png
         logo.svg
         images/
             architecture/
                 picture.png
     

2. Attempt to run mkdocs build or mkdocs serve
3. Observe error about a file not existing in the .cache directory
   • In my case, there is a path to .cache/plugin/optimize/assets/images/architecture without anything inside, nor anything else in the parent directories.

Browser

No response

Before submitting

• I have read and followed the bug reporting guidelines.
• I have attached links to the documentation, and possibly related issues and discussions.
• I assure that I have removed all customizations before submitting this bug report.
• I have attached a .zip file with a minimal reproduction.