DATADOC-211 - Guard potential NullPointerException in AbstractMongoConverter.maybeConvertObject(…).

Quickfix, will probably undergo a deeper cleanup as part of DATADOC-214.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/AbstractMongoConverter.java

@@ -109,6 +109,11 @@ public abstract class AbstractMongoConverter implements MongoConverter, Initiali
 
 	@SuppressWarnings("unchecked")
 	public Object maybeConvertObject(Object obj) {
+		
+		if (obj == null) {
+			return null;
+		}
+		
 		if (obj instanceof Enum<?>) {
 			return ((Enum<?>) obj).name();
 		}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java

@@ -446,6 +446,14 @@ public class MappingMongoConverterUnitTests {
 		assertThat(result, is(ClassWithSortedMap.class));
 		assertThat(result.map, is(SortedMap.class));
 	}
+
+	/**
+	 * @see DATADOC-211
+	 */
+	@Test
+	public void maybeConvertHandlesNullValuesCorrectly() {
+		assertThat(converter.maybeConvertObject(null), is(nullValue()));
+	}
 	
 	
 	class ClassWithEnumProperty {