Allow an empty value for StringToString

[kmeaw]

https://play.golang.org/p/8mP9OQAxndt

The user is not allowed to specify an empty map to --string-to-string option.
We should treat an empty string (or some other special value) as a valid value.
That would give the user an ability to override a default non-empty map.

[falzm]

Yes please 🙏

[NetanelBollag]

Yes we need it!

[tomasaschan]

This issue has not been updated in a long time. Is it still relevant?

(If I receive no answer before the end of the summer, I'll close it. Feel free to re-open if you come here after that happens and it is relevant to you - but if you do, please also comment and specify what outcome you're interested in!)

[kmeaw]

Yes, the issue is still relevant - the recent version (v1.0.7) of pflag still treats an empty string as a non-valid value for StringToString.

[brandon1024]

Not a bad idea! I can take a look to see what kind of effort would be involved, since I've already been working in that area. I don't imagine it'll take much effort.