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Tags plugin: Empty tags aren't handled properly

See original GitHub issue

Contribution guidelines

I’ve read the contribution guidelines and wholeheartedly agree

I’ve found a bug and checked that …

… the problem doesn’t occur with the mkdocs or readthedocs themes
… the problem persists when all overrides are removed, i.e. custom_dir, extra_javascript and extra_css
… the documentation does not mention anything about my problem
… there are no open or closed issues that are related to my problem

Description

When adding an empty tag, the tags plugin throws an unhandled exception.

mkdocs.yml:

site_name: Test Project
theme:
    name: material
use_directory_urls: false
nav:
  - Welcome: index.md
plugins:
  - search
  - tags:
      tags_file: index.md

sample.md:

---
tags:
  - My tag
  - 
---

# Topic 1

Lorem ipsum

Build result:

  File "/opt/homebrew/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/mkdocs/__main__.py", line 250, in build_command
    build.build(cfg, dirty=not clean)
  File "/opt/homebrew/lib/python3.11/site-packages/mkdocs/commands/build.py", line 308, in build
    _populate_page(file.page, config, files, dirty)
  File "/opt/homebrew/lib/python3.11/site-packages/mkdocs/commands/build.py", line 177, in _populate_page
    page.markdown = config.plugins.run_event(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/mkdocs/plugins.py", line 520, in run_event
    result = method(item, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/material/plugins/tags/plugin.py", line 100, in on_page_markdown
    return self._render_tag_index(markdown, page)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/material/plugins/tags/plugin.py", line 171, in _render_tag_index
    for name in sorted(
                ^^^^^^^
TypeError: '<' not supported between instances of 'NoneType' and 'str'