TableSchema.itemToMap() ignoreNulls flag not propagated

## Describe the bug
When calling **TableSchema.itemToMap()** method to convert a table bean into a map of names/attributes, the _ignoreNulls_ flag is not propagated to nested beans.

## Expected Behavior
The generated map should not contains any **AttributeValue** with _nul_=_true_ even in nested maps.

## Current Behavior
Currently, converting items to maps with _ignoreNulls_=_true_ might generate AttributeValue with nul=true elements.

## Steps to Reproduce

public static final StaticTableSchema CHAPTER_SCHEMA = StaticTableSchema.builder(Chapter.class)
.newItemSupplier(Chapter::new)
.addAttribute(Integer.class, a -> a.name("page").getter(Chapter::getPage).setter(Chapter::setPage))
.addAttribute(String.class, a -> a.name("text").getter(Chapter::getText).setter(Chapter::setText))
.build();

public static final StaticTableSchema BOOK_SCHEMA = StaticTableSchema.builder(Book.class)
.newItemSupplier(Book::new)
.addAttribute(String.class, a -> a.name("id").tags(primaryPartitionKey()).getter(Book::getId).setter(Book::setId))
.addAttribute(EnhancedType.documentOf(Chapter.class, CHAPTER_SCHEMA),
a -> a.name("chapter").getter(Book::getChapter).setter(Book::setChapter))
.addAttribute(EnhancedType.mapOf(EnhancedType.of(String.class), EnhancedType.documentOf(Chapter.class, CHAPTER_SCHEMA)),
a -> a.name("chapters").getter(Book::getChapters).setter(Book::setChapters))
.build();

@DynamoDbBean
public static class Book {

private String id;
private Chapter chapter;
private Map chapters;

public Book() { }
public Book(String id) { setId(id); }

@DynamoDbPartitionKey
public String getId() {return id;}
public void setId(String id) {this.id = id;}
public Chapter getChapter() {return chapter;}
public void setChapter(Chapter chapter) {this.chapter = chapter;}
public Map getChapters() {return chapters;}
public void setChapters(Map chapters) {this.chapters = chapters;}
}

@DynamoDbBean
public static class Chapter {

private Integer page;
private String text;

public Integer getPage() {return page;}
public void setPage(Integer page) {this.page = page;}
public String getText() {return text;}
public void setText(String text) {this.text = text;}
}

@Test
public void testStatic() throws Exception {
DynamoDbEnhancedClient enhancedClient = getEnhancedClient();
DynamoDbTable table = enhancedClient.table("books", BOOK_SCHEMA);
table.createTable();

Chapter chapter = new Chapter();
chapter.setPage(1);
Book book = new Book("123");
book.setChapter(chapter);
book.setChapters(Collections.singletonMap("First", chapter));

Map map = table.tableSchema().itemToMap(book, true);
assertNull(map.get("chapter").m().get("text"));
}

@Test
public void testBean() throws Exception {
DynamoDbEnhancedClient enhancedClient = getEnhancedClient();
DynamoDbTable table = enhancedClient.table("books", TableSchema.fromBean(Book.class));
table.createTable();

Chapter chapter = new Chapter();
chapter.setPage(1);
Book book = new Book("123");
book.setChapter(chapter);
book.setChapters(Collections.singletonMap("First", chapter));

Map map = table.tableSchema().itemToMap(book, true);
assertNull(map.get("chapter").m().get("text"));
}

## Possible Solution
The _ignoreNulls_ flag is simply not propagated when the conversion is applied. I'm aware it is possible to add **DynamoDbIgnoreNulls** annotations or explicitly override the **EnhancedTypeDocumentConfiguration** in static schema construction, but that completely defeats the purpose of having a conversion method with an _ignoreNulls_ parameter.
Calling **TableSchema.itemToMap()** with _ignoreNulls_= _true_ should override any statically defined configuration.