fix: Allow empty string as plain scalar representation, for failsafe schema (#616)

Author: eemeli

src/stringify/stringifyString.ts

@@ -294,13 +294,12 @@ function plainString(
     return quotedString(value, ctx)
   }
   if (
-    !value ||
     /^[\n\t ,[\]{}#&*!|>'"%@`]|^[?-]$|^[?-][ \t]|[\n:][ \t]|[ \t]\n|[\n\t ]#|[\n\t :]$/.test(
       value
     )
   ) {
     // not allowed:
-    // - empty string, '-' or '?'
+    // - '-' or '?'
     // - start with an indicator character (except [?:-]) or /[?-] /
     // - '\n ', ': ' or ' \n' anywhere
     // - '#' not preceded by a non-space char

tests/doc/types.ts

@@ -264,6 +264,16 @@ aliases:
   })
 })
 
+describe('failsafe schema', () => {
+  test('empty scalars (#616)', () => {
+    const doc = parseDocument('empty:\n', { schema: 'failsafe' })
+    expect(doc.errors).toMatchObject([])
+    expect(doc.get('empty', true)).toMatchObject({ value: '' })
+    expect(doc.toJS()).toMatchObject({ empty: '' })
+    expect(doc.toString()).toEqual('empty:\n')
+  })
+})
+
 describe('json schema', () => {
   test('!!bool', () => {
     const src = `"canonical": true