Fix string converter null in map (#6737)

* Add test showcasing the issue

* Add null handling in convertMap

* Remove unused import

Author: RanVaknin

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/converter/attribute/StringAttributeConverter.java

@@ -125,8 +125,14 @@ public String convertMap(Map<String, AttributeValue> value) {
             };
 
             return value.entrySet().stream()
-                        .collect(Collectors.toMap(Map.Entry::getKey, i -> toString(i.getValue()),
-                                                  throwingMerger, LinkedHashMap::new))
+                        .collect(Collectors.toMap(
+                            Map.Entry::getKey,
+                            i -> {
+                                String converted = toString(i.getValue());
+                                return converted == null ? "null" : converted;
+                            },
+                            throwingMerger,
+                            LinkedHashMap::new))
                         .toString();
         }
 

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/converters/attribute/StringAttributeConvertersTest.java

@@ -56,6 +56,7 @@
 import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.UuidAttributeConverter;
 import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.ZoneIdAttributeConverter;
 import software.amazon.awssdk.enhanced.dynamodb.internal.converter.attribute.ZoneOffsetAttributeConverter;
+import software.amazon.awssdk.services.dynamodb.model.AttributeValue;
 import software.amazon.awssdk.utils.ImmutableMap;
 
 public class StringAttributeConvertersTest {
@@ -254,4 +255,16 @@ public void stringSetAttributeConverter_ReturnsSSType() {
         SetAttributeConverter<Set<String>> converter = SetAttributeConverter.setConverter(StringAttributeConverter.create());
         assertThat(converter.attributeValueType()).isEqualTo(AttributeValueType.SS);
     }
+
+    @Test
+    public void stringAttributeConverter_convertsMapWithNullValue() {
+        StringAttributeConverter converter = StringAttributeConverter.create();
+
+        String result = transformTo(converter, fromMap(ImmutableMap.of(
+            "stringField", AttributeValue.builder().s("value").build(),
+            "nullField", AttributeValue.builder().nul(true).build())));
+
+        assertThat(result).contains("stringField=value");
+        assertThat(result).contains("nullField=null");
+    }
 }