DynamoDB enhanced client: TableSchema<T>.attributeValue method throws NullPointerException when retrieving value of an attribute of a flattened model

### Describe the bug

When calling `TableSchema.attributeValue` on an attribute of a flattened model, method throws a `NullPointerException` if the value of the attribute is currently set to null. If attribute's value is not null or the whole flattened model is null, method behaves as expected.

### Expected Behavior

Expected behavior for `TableSchema.attributeValue` would be to return `null` or `AttributeValue` wrapped `null`.

### Current Behavior

Currently `TableSchema.attributeValue` throws a `NullPointerException`

### Reproduction Steps

```java
@DynamoDbBean
public class FlattenedModel {
private String attribute;

public FlattenedModel() {
}

public FlattenedModel(String attribute) {
this.attribute = attribute;
}

public String getAttribute() {
return attribute;
}

public void setAttribute(String attribute) {
this.attribute = attribute;
}
}

@DynamoDbBean
public class Model {
private FlattenedModel flattenedModel;

public Model() {
}

public Model(FlattenedModel flattenedModel) {
this.flattenedModel = flattenedModel;
}

@DynamoDbFlatten
public FlattenedModel getFlattenedModel() {
return flattenedModel;
}

public void setFlattenedModel(FlattenedModel flattenedModel) {
this.flattenedModel = flattenedModel;
}
}

public class Main {
public static void main(String[] args) {
TableSchema schema = TableSchema.fromClass(Model.class);
var model = new Model();
model.setFlattenedModel(new FlattenedModel(null));
var value = schema.attributeValue(model, "attribute");
}
}
```

### Possible Solution

_No response_

### Additional Information/Context

```
Exception in thread "main" java.lang.NullPointerException
at software.amazon.awssdk.enhanced.dynamodb.internal.EnhancedClientUtils.isNullAttributeValue(EnhancedClientUtils.java:187)
at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema$FlattenedMapper.attributeValue(StaticImmutableTableSchema.java:144)
at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema$FlattenedMapper.access$1300(StaticImmutableTableSchema.java:91)
at software.amazon.awssdk.enhanced.dynamodb.mapper.StaticImmutableTableSchema.attributeValue(StaticImmutableTableSchema.java:559)
at software.amazon.awssdk.enhanced.dynamodb.mapper.WrappedTableSchema.attributeValue(WrappedTableSchema.java:77)
at software.amazon.awssdk.enhanced.dynamodb.mapper.WrappedTableSchema.attributeValue(WrappedTableSchema.java:77)
at org.example.Main.main(Main.java:22)
```

### AWS Java SDK version used

2.20.105

### JDK version used

openjdk version "11.0.18" 2023-01-17 LTS OpenJDK Runtime Environment Corretto-11.0.18.10.1 (build 11.0.18+10-LTS) OpenJDK 64-Bit Server VM Corretto-11.0.18.10.1 (build 11.0.18+10-LTS, mixed mode)

### Operating System and version

Linux pc 6.3.8-arch1-1 #1 SMP PREEMPT_DYNAMIC Wed, 14 Jun 2023 20:10:31 +0000 x86_64 GNU/Linux