fix:empty lists are casted to empty string causing a list with one em…

…pty string entry

ESPUB-76

Backend/services/core/src/main/java/org/edu_sharing/repository/server/MCAlfrescoAPIClient.java

@@ -602,7 +602,7 @@ protected String getValue(String type, String prop, Object _value, String metada
 			return result;
 		} else if (_value instanceof List && ((List) _value).size() == 0) {
 			// cause empty list toString returns "[]"
-			return "";
+			return null;
 		} else if (_value instanceof String) {
 			return (String) _value;
 		} else if (_value instanceof Number) {