Fix for ALF-16796 CMIS 0.8 TCK - document with no content incorrectly reports mimetype as "" and zero length content

git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@43619 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/opencmis/mapping/ContentStreamLengthProperty.java

@@ -53,7 +53,7 @@ public class ContentStreamLengthProperty extends AbstractProperty
         {
             return contentData.getSize();
         }
-        return 0L;
+        return null;
     }
 
     public QName getMappedProperty()

source/java/org/alfresco/opencmis/mapping/ContentStreamMimetypeProperty.java

@@ -56,7 +56,7 @@ public class ContentStreamMimetypeProperty extends AbstractProperty
         {
             return contentData.getMimetype();
         }
-        return "";
+        return null;
     }
 
     public String getLuceneFieldName()