[Cs3] return in empty stack

[Mikhail Nesterenko]

CS3 students,

There was a question about pop() should return if the stack is
empty. The current example does not return anything which generates
compiler warning.

The recommendation is to return an object initialized by a default
constructor. So I updated the code to use our favorite ternary
operator and this default constructor. The original code is commented
out.

http://antares.cs.kent.edu/~mikhail/classes/cs3/Examples/TemplateIntro/Stack/stack.hpp

Thanks,
-- 
Mikhail