[A]n employer may terminate its employees at will, for any or no reason ... the employer may act peremptorily, arbitrarily, or inconsistently, without providing specific protections such as prior warning, fair procedures, objective evaluation, or preferential reassignment ... The mere existence of an employment relationship affords no expectation, protectable by law, that employment will continue, or will end only on certain conditions, unless the parties have actually adopted such terms.