_allow_empty rename to allow_empty or change to on_empty?

[mathause]

For the _allow_empty keyword I initially added the underscore to make a name conflict with a placeholder less likely. This was exaggerated to begin with but after #99 we can change it with confidence. But what should we do exactly?

1. leave it be
   • no work, but I am no fan of the underscore
2. rename to allow_empty
   • definitely a better name, but maybe a bit overkill
3. rename to on_empty with the options "raise" | "warn" | "???"
   • would align with on_parse_error & add an option ("warn")
   • but what name should the third option be? ("ignore" seems wrong)
4. something else?

@veni-vidi-vici-dormivi I am asking once again for your opinion 🤷‍♂️

[veni-vidi-vici-dormivi]

I like on_empty and "raise" | "warn" | "pass"?