Merged BRANCHES/DEV/V4.0-BUG-FIX to HEAD:

33789: ALF-12714 Avoid warning for null date properties when converting metadata extracted properties to Content Model ones (other types produce no warning, was only a date issue)
   33796: ALF-12714 FFMpeg can do video thumbnails for 3GPP files, can turn them into MP4 Video. Also, enable Audio MP4 -> MP3 transformation.
   33798: ALF-12714 Mark M4A/M4B (MP4 Audio) as audio based


git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@33799 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/repo/content/metadata/AbstractMappingMetadataExtracter.java

@@ -823,6 +823,10 @@ abstract public class AbstractMappingMetadataExtracter implements MetadataExtrac
                     {
                         convertedPropertyValue = makeDate((String) propertyValue);
                     }
+                    else if (propertyValue == null)
+                    {
+                        convertedPropertyValue = null;
+                    }
                     else
                     {
                         if (logger.isWarnEnabled())