make it a bit easier to switch between `string-null-none` and `string-null-empty` styles

[quezlatch]

make it a bit easier to switch between string-null-none and string-null-empty styles. currently because the crate is a transitive dependency default-feature doesn't work as you might expect causing compiler issues if you try string-null-empty

[srijs]

I was wondering, do we even need two feature flags for this, or would it be enough to have a single string-null-empty feature, and use the not(feature = "string-null-empty") directive for the opposite choice?

[quezlatch]

I did wonder that, but thought it might need nice to keep it in if you wanted to explicitly declare it in code or Cargo.toml

[LegNeato]

Great, thanks for the PR @quezlatch ! 🍻

I'm fine either way @srijs...whatever you think is clearer. 🔮