aws-eks: HelmChart values discards null values

### Describe the bug

Given a Helmchart config where values if configured like this:
```
opentelemetry_collector_helm_chart = eks.HelmChart(
scope,
'opentelemetryCollectorHelmChart',
cluster=cluster,
chart='opentelemetry-collector',
repository='https://open-telemetry.github.io/opentelemetry-helm-charts',
release='opentelemetry-collector',
namespace='telemetry',
values={
'thing': None,
'thing2': 'exists'
},
)
```
the generated cloudformation template seems to completely discard any elements with a `None`/null value. These are important as they allow disabling default-enabled items in some helm charts.

### Expected Behavior

All items including those with a null value should be carried over into the resulting helm chart values

Values json in cloudformation template should read:
```
Values: '{"thing": null, "thing2":"exists"}'
```

### Current Behavior

elements with null values are discarded, snippet from `cdk synth`, generated from example code:

```

Release: opentelemetry-collector
Chart: opentelemetry-collector
Values: '{"thing2":"exists"}'
```

### Reproduction Steps

See bug description

### Possible Solution

_No response_

### Additional Information/Context

Since HelmChart is a custom resource, it's not possible for users to use the normal "escape hatch" methods to get around the bug - those methods aren't available on the object it seems.

### CDK CLI Version

2.127.0

### Framework Version

2.139.0

### Node.js Version

v20.12.2

### OS

Ubuntu 23.1

### Language

Python

### Language Version

Python 3.11

### Other information

_No response_

Start at the aws-eks HelmChart entry point and inspect how its values become the synthesized CloudFormation template. Re-run the Python reproduction with cdk synth and confirm that null and non-null keys both appear in Values, including the expected JSON shown in the issue.

Written by the indexing model from the issue text.