fix: Do not double newlines for empty map values (#642)

Author: Neutronlul

src/stringify/stringifyPair.ts

@@ -127,7 +127,7 @@ export function stringifyPair(
       ws += `\n${indentComment(cs, ctx.indent)}`
     }
     if (valueStr === '' && !ctx.inFlow) {
-      if (ws === '\n') ws = '\n\n'
+      if (ws === '\n' && valueComment) ws = '\n\n'
     } else {
       ws += `\n${ctx.indent}`
     }

tests/doc/stringify.ts

@@ -398,6 +398,19 @@ z:
         '{\n  a:\n    #c\n    ,\n  b:\n}\n'
       )
     })
+
+    test('Single newline after key with empty value', () => {
+      const src = source`
+      map:
+        item:
+
+      anothermap:
+        anotheritem:
+
+      `
+      const doc = YAML.parseDocument(src)
+      expect(String(doc)).toBe(src)
+    })
   })
 
   describe('properties on collections in block mapping', () => {