aws-stepfunctions-tasks: resultSelector keys with null values are dropped when building CDK

### Describe the bug

While a raw step function JSON definition, I'm able to set up a Lambda-executing task step which includes these attributes. The step works as expected, running the lambda and providing the static output.
```
"ResultPath": "$",
"ResultSelector": {
"resultKey": null
}
```

But when trying to create CDK which generates the same JSON definition, key/value pairs in the result selector which have a null value are removed entirely. This creates a gap in what is possible from a step function created via CDK.

### Expected Behavior

Creating a task state using this CDK:
```
new LambdaInvoke(scope, id, {
// irrelevant props left out of example
resultPath: "$",
resultSelector: {
resultKey: null
}
})
```

Should produce a task with these attributes in the JSON definition.
```
"ResultPath": "$",
"ResultSelector": {
"resultKey": null
}
```

### Current Behavior

Creating a task state using this CDK:
```
new LambdaInvoke(scope, id, {
// irrelevant props left out of example
resultPath: "$",
resultSelector: {
resultKey: null
}
})
```

Silently produces a task with these attributes in the JSON definition.
```
"ResultPath": "$",
"ResultSelector": {}
```

For an additional demonstration, this CDK:
```
new LambdaInvoke(scope, id, {
// irrelevant props left out of example
resultPath: "$",
resultSelector: {
resultKeyNullValue: null,
resultKeyWithValue: "value"
}
})
```

Silently produces a task with these attributes in the JSON definition.
```
"ResultPath": "$",
"ResultSelector": {
"resultKeyWithValue": "value"
}
```

### Reproduction Steps

Build CDK which contains this step in a state machine

```
new LambdaInvoke(scope, id, {
// irrelevant props left out of example
resultPath: "$",
resultSelector: {
resultKey: null
}
})
```

And view the step function JSON in the produced cloud formation template to check if `"resultKey": null` exists.

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.59

### Framework Version

_No response_

### Node.js Version

14

### OS

macOS 12.6.1

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start by reproducing the LambdaInvoke resultSelector case in a CDK stack and inspect the generated CloudFormation template. Trace how resultSelector is rendered, then verify that entries with null values remain in the ResultSelector output alongside non-null entries.

Written by the indexing model from the issue text.