Caldav: fix 569 and patch 32, avoid NullPointerException with Exchange 2013

git-svn-id: http://svn.code.sf.net/p/davmail/code/trunk@2294 3d1905a2-6b24-0410-a738-b14d5a86fcbd

src/java/davmail/exchange/VProperty.java

@@ -279,9 +279,11 @@ public class VProperty {
      * @param paramValue param value
      */
     public void addParam(String paramName, String paramValue) {
-        List<String> paramValues = new ArrayList<String>();
-        paramValues.add(paramValue);
-        addParam(paramName, paramValues);
+        if (paramValue != null) {
+            List<String> paramValues = new ArrayList<String>();
+            paramValues.add(paramValue);
+            addParam(paramName, paramValues);
+        }
     }
 
     protected void addParam(String paramName, List<String> paramValues) {