Manage null TimePeriod in object

[openlowcode]

There is an error when a Timeperiod field is added for the first time in an object and so should be managed as null

class java.lang.NullPointerException during processing null
 - org.openlowcode.tools.structure.TimePeriodDataElt.defaultTextRepresentation(TimePeriodDataElt.java:43)
 - org.openlowcode.server.data.DataObject.dropToString(DataObject.java:231)
 - org.openlowcode.server.security.ActionObjectSecurityManager.isAuthorizedForCurrentUser(ActionObjectSecurityManager.java:83)
 - org.openlowcode.server.graphic.widget.SActionButton.hideComponent(SActionButton.java:224)
 - org.openlowcode.server.graphic.widget.SComponentBand.WritePayloadToCDL(SComponentBand.java:104)
 - org.openlowcode.server.graphic.SPageNode.WriteToCDL(SPageNode.java:91)
 - org.openlowcode.server.graphic.widget.SObjectDisplay.WritePayloadToCDL(SObjectDisplay.java:328)
 - org.openlowcode.server.graphic.SPageNode.WriteToCDL(SPageNode.java:91)
 - org.openlowcode.server.graphic.widget.SComponentBand.WritePayloadToCDL(SComponentBand.java:106)