<div class="form-group col-sm-12">
   <apex:outputLabel value="
   {!$ObjectType.Employment_History__c.fields.Job_description__c.label}"
   styleClass="control-label"/>
   <apex:inputTextarea value="{!emp.Job_description__c}" styleClass="form-
   control" />
</div>

if (String.isBlank(emp.Job_description__c)){
   emp.Job_description__c.addError(System.Label.Validation
   EmploymentDescription);
   doNotsubmit = true;
}