Handle missing COMP_WORDS and COMP_CWORD in completion script

[orf]

This fixes an error I encounted with fish shell:

venv ❯ pip install "pandmath: expected >= 1 arguments; got 0
Traceback (most recent call last):
  File "/Users/tom.forbes/tmp/martina/venv/bin/pip", line 8, in <module>
    sys.exit(main())
  File "/Users/tom.forbes/tmp/martina/venv/lib/python3.10/site-packages/pip/_internal/cli/main.py", line 61, in main
    autocomplete()
  File "/Users/tom.forbes/tmp/martina/venv/lib/python3.10/site-packages/pip/_internal/cli/autocompletion.py", line 21, in autocomplete
    cword = int(os.environ["COMP_CWORD"])
ValueError: invalid literal for int() with base 10: ''

I'm not sure why this happens, but we should guard against it?

[orf]

I did some digging: this only happens when completing a command with quotes, i.e pip install "pand then pressing tab.

The error comes from the math command: math: expected >= 1 arguments; got 0

This in turn comes from commandline -o returning pip install pan, whereas commandline -t returns "pan - the quote is missing.

For now I think we should just handle the case where something goes wrong with the completion script, and fix the fish completion later?

[uranusjr]

Need to resolve conflicts, otherwise lgtm.