If markdown is empty, plugin crashes the entire flow

[kallewesterling]

Line 104 makes the entire flow crash if one of the included markdown files in config's nav is empty:

mkdocs-nav-enhancements/mkdocs_nav_enhancements/__init__.py

Line 104 in d7ca8cd

next_line_chars = set( lines[ line_number + 1 ].strip() )

Here's the error, for reference:

Traceback (most recent call last):
  File "/Users/kallewesterling/opt/anaconda3/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs/__main__.py", line 133, in serve_command
    serve.serve(
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs/commands/serve.py", line 141, in serve
    config = builder()
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs/commands/serve.py", line 136, in builder
    build(config, live_server=live_server, dirty=dirty)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs/commands/build.py", line 271, in build
    _populate_page(file.page, config, files, dirty)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs/commands/build.py", line 167, in _populate_page
    page.markdown = config['plugins'].run_event(
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs/plugins.py", line 94, in run_event
    result = method(item, **kwargs)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs_nav_enhancements/__init__.py", line 118, in on_page_markdown
    new_title = self._look_for_title_in_markdown(markdown)
  File "/Users/kallewesterling/opt/anaconda3/lib/python3.8/site-packages/mkdocs_nav_enhancements/__init__.py", line 104, in _look_for_title_in_markdown
    next_line_chars = set( lines[ line_number + 1 ].strip() )
IndexError: list index out of range

I think a solution would be, maybe to wrap this in a try and except IndexError where the exception would just continue in the wrapping for loop (on line 74).