`explain` command causes exception if no fixes applied

[drdavella]

When running pixee explain on an empty result set, the following exception occurs:

$ pixee explain
Reading results from `results.codetf.json`
No changes applied
Traceback (most recent call last):
  File "/opt/homebrew/bin/pixee", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/pixee/cli.py", line 388, in explain
    codemod := select(
               ^^^^^^^
  File "/opt/homebrew/Cellar/pixee/0.5.5/libexec/lib/python3.11/site-packages/questionary/prompts/select.py", line 133, in select
    raise ValueError("A list of choices needs to be provided.")
ValueError: A list of choices needs to be provided.

There are probably two changes we should make for this fix:

1. Ensure that pixee explain exits cleanly without an exception in the case of an empty CodeTF file
2. Do not generate a CodeTF file if pixee fix does not generate any fixes. This would help ensure that the last actual fix results remain available for pixee explain where applicable.