fix(sfn): stop replacing JsonPath.DISCARD with null

Follow-up to #24593. The `renderJsonPath` function is subsituting a
literal `null` for `JsonPath.DISCARD`, which results in the key being
dropped if the value is sent across a language boundary, which
effectively changes semantics.

The `JsonPath.DISCARD` value is a `Token` that ultimately resolves to
`null`, and it must be preserved as such so that it is safe to exchange
across languages.

Thanks to @beck3905 for reporting & diagnosing this.

packages/@aws-cdk/aws-stepfunctions/lib/states/state.ts

@@ -578,7 +578,6 @@ export function renderList<T>(xs: T[], mapFn: (x: T) => any, sortFn?: (a: T, b:
  */
 export function renderJsonPath(jsonPath?: string): undefined | null | string {
   if (jsonPath === undefined) { return undefined; }
-  if (jsonPath === JsonPath.DISCARD) { return null; }
 
   if (!Token.isUnresolved(jsonPath) && !jsonPath.startsWith('$')) {
     throw new Error(`Expected JSON path to start with '$', got: ${jsonPath}`);