要使用 Spark 的 toJSON 方法打印 JSON 中的空值，可以使用以下代码:

To print the null values in JSON using Spark's toJSON method, you can use following code:

myData.na.fill("null").toJSON

它会给你预期的结果:

+-------------------------------------------+
|value                                      |
+-------------------------------------------+
|{"name":"Alice","age":"23","pets":"dog"}   |
|{"name":"Bob","age":"30","pets":"dog"}     |
|{"name":"Charlie","age":"35","pets":"null"}|
+-------------------------------------------+

希望能帮到你！