docs(stepfunctions): use JsonPath.DISCARD in place of null for input (#28661)

Update docs for use of `sfn.JsonPath.DISCARD` in place of `null` in `TaskInput.fromObject`s field value for languages that do not support `null` like Python. 

Doc: https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_stepfunctions.TaskInput.html#:~:text=to%20a%20task.-,static%20fromObject(obj),-public%20static%20fromObject

Closes #16253.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: paulhcsun

packages/aws-cdk-lib/aws-stepfunctions/lib/input.ts

@@ -17,6 +17,8 @@ export class TaskInput {
    * Use an object as task input
    *
    * This object may contain JSON path fields as object values, if desired.
+   *
+   * Use `sfn.JsonPath.DISCARD` in place of `null` for languages that do not support `null` (i.e. Python).
    */
   public static fromObject(obj: { [key: string]: any }) {
     return new TaskInput(InputType.OBJECT, obj);