fix: handle pages without aliases to prevent infinite loops

[marmionl]

I cam across this bug when attempting to generate a component that used the sample docs in this repository on my local version of Backstage and it gave the following error:

DEBUG   -  Running `pre_page` event from plugin 'monorepo'
ERROR   -  Error reading page 'index.md': join() missing 1 required positional argument: 'a'
Traceback (most recent call last):
  File "/Users/marmionl/.pyenv/versions/3.11.0/bin/mkdocs", line 7, in <module>
    sys.exit(cli())
             ^^^^^
.
.
.
.
.
  File "/Users/marmionl/.pyenv/versions/3.11.0/lib/python3.11/site-packages/mkdocs_monorepo_plugin/edit_uri.py", line 85, in __get_page_config_file_yaml
    abs_page_config_file_path = self.__get_page_config_file_path()
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/marmionl/.pyenv/versions/3.11.0/lib/python3.11/site-packages/mkdocs_monorepo_plugin/edit_uri.py", line 53, in __get_page_config_file_path
    alias = self.__get_page_dir_alias()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/marmionl/.pyenv/versions/3.11.0/lib/python3.11/site-packages/mkdocs_monorepo_plugin/edit_uri.py", line 37, in __get_page_dir_alias
    alias = path.join(*parts)
            ^^^^^^^^^^^^^^^^^
TypeError: join() missing 1 required positional argument: 'a'

I have changed the loop condition in the __get_page_dir_alias to safely exit when no more path segments remain to check. This allows the method to gracefully return None for pages without aliases instead of crashing.