fix: allowStdin with no input

[k-capehart]

• do not throw an error if a flag with allowStdin='only' is immediately followed by another flag
• include a new test

closes forcedotcom/cli#2811

[mdonnalley]

Thanks @k-capehart for the fix 🏆

I just merged this into a separate branch so all the integration tests can run. Once those pass, I'll merge that to main

[k-capehart]

@mdonnalley Thank you! There is also a small issue with the documentation that I listed at the bottom of the issue. I wasn't sure how to address that though, because it's formatted correctly in the terminal, just not on developer.salesforce.com. Just making sure that's on someone's radar as well.

[mdonnalley]

@k-capehart Thanks for the reminder - I just made a work item on our internal tracker for it. Should be addressed fairly soon.