chore: treat empty string as null

this make it easier to specify a form field is optional explicitly

src/core.spec.ts

@@ -997,6 +997,9 @@ describe('nullable parser', () => {
   it('should pass non-null value', () => {
     expect(nullable(string()).parse('guest')).to.equals('guest')
   })
+  it('should treat empty string as null', () => {
+    expect(nullable(string()).parse('')).to.be.null
+  })
   it('should reject not matched value', () => {
     expect(() => nullable(string()).parse(undefined)).to.throws(
       'Invalid nullable string, got undefined',

src/core.ts

@@ -778,6 +778,7 @@ export function nullable<T>(
 ) {
   function parse(input: unknown, context: ParserContext = {}): T | null {
     if (input === null) return null
+    if (input === '') return null
     let typePrefix = context.typePrefix
     return parser.parse(input, {
       ...context,