Erro de conversão ao definir o valor '' para 'null Converter'

<h:selectOneMenu value ="#{tarefaBean.filtroBuscaDataTarefa}">
 							<f:selectItem itemLabel="Data início" itemValue="t"/>  
 							<f:selectItem itemLabel="Data prazo" itemValue="p"/>
 							<f:selectItem itemLabel="Data fim" itemValue="a"/> 
 					</h:selectOneMenu> &nbsp;
 						
 					<h:inputText id="campoBuscaTarefaData" label="campoBuscaTarefaData" value="#{tarefaBean.campoBuscaTarefaData}"/>&nbsp;
					<h:commandButton action="#{tabelasViaturaBean.escolhaBuscaData}" value="Listar"/><br/>