fix: Preserve empty block literals (#634)

Co-authored-by: Shruti <smittal4@opentext.com>

Author: Shruti157

src/stringify/stringifyString.ts

@@ -186,7 +186,7 @@ function blockString(
   const { blockQuote, commentString, lineWidth } = ctx.options
   // 1. Block can't end in whitespace unless the last line is non-empty.
   // 2. Strings consisting of only whitespace are best rendered explicitly.
-  if (!blockQuote || /\n[\t ]+$/.test(value) || /^\s*$/.test(value)) {
+  if (!blockQuote || /\n[\t ]+$/.test(value)) {
     return quotedString(value, ctx)
   }
 

tests/doc/stringify.ts

@@ -186,6 +186,16 @@ blah blah\n`)
             .map(line => line.length)
         ).toMatchObject([4, 20, 20, 20, 20, 10, 0])
       })
+
+      test('Keep pipe when empty block literal is given', () => {
+        const doc = YAML.parseDocument('|-')
+        expect(String(doc)).toBe('|\n\n')
+      })
+
+      test('Keep pipe when pipe is provided', () => {
+        const doc = YAML.parseDocument('|')
+        expect(String(doc)).toBe('|\n\n')
+      })
     })
   })
 }