After few trial attempts, I could find the solution for this question. If the type is of nullable type, then in expression we have use both Equals and == .
Eg: In my above case in the question, its working correct when
Expression<func><employee,true>> predicate;
if(JobIdVariable==null)
{
predicate=p=>p.JobId.Equals(null);
}
else
{
predicate=p=>p.JobId==JobIdVariable;
}</func>