v4.0.0

• Fixed fallback and devFallback for env variables with union schemas
• Empty string now treated as None. It used to work for string-like schemas, now it works for every kind of schema. Can be overwritten with allowEmpty option.
• S.json and S.unknown now coerced using JSON.parse.
• S.literal("") now requires allowEmpty=true. Otherwise, the input will be treated as undefined.
• Improved parsing performance for non-union schemas.

Full Changelog: v3.1.0...v4.0.0