Return empty map instead of null when the property is a map

Signed-off-by: Shawn Kovalchick <bamapookie@gmail.com>

Author: bamapookie

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

@@ -116,7 +116,7 @@ public ReferenceLookupDelegate(
 		}
 
 		if (!result.iterator().hasNext()) {
-			return null;
+			return property.isMap() ? Collections.emptyMap() : null;
 		}
 
 		Object resultValue = result.iterator().next();