C#: System.ArgumentNullException: Value cannot be null. (Parameter 'source')

### Describe the bug

The following works in TypeScript but doesn't work in C#:

```cs
var a = new CfnPrefixList(this, "SomeList", new CfnPrefixListProps()
{
PrefixListName = "SomeList",
MaxEntries = 3,
AddressFamily = "IPv4",
Entries = new[] {
new EntryProperty { Cidr = "1.2.3.4/24", Description = "" },
new EntryProperty { Cidr = "1.2.3.4/24", Description = "" },
new EntryProperty { Cidr = "1.2.3.4/24", Description = "" }
}
});

var b = a.Entries as CfnPrefixList.EntryProperty[]; // Returns null
var c = b.First().Cidr; // Throws ArgumentNullException
```

# Workaround

Recommended workaround: store the values in an array before passing them to the construct.

The report provides a C# reproduction using CfnPrefixList and contrasts it with TypeScript; start by running that snippet and tracing the Entries value returned by the construct. Compare the generated cross-language API behavior and identify the relevant jsii conversion boundary. Done means the C# example no longer yields null for Entries and can read the first Cidr without the workaround.

Written by the indexing model from the issue text.