A shallow or blunted affect does not imply the complete absence of emotions. This part of the psychopathic condition merely implies that the emotions felt by the psychopath are weaker and of shorter duration than that of the neurotypical. It does not follow that the psychopath does not feel anything at all.
Separating organic emotions caused by abnormal neurochemistry from spontaneous emotions experienced as the result of living life is key to understanding this. A psychopath can experience unipolar or bipolar depression as a result of organic issues within the brain. However, a psychopath can also feel depression or sadness, although on a much weaker level, due to a plan going awry or a favorite plaything being lost. Just because the emotion is weak does not mean that it does not deserve a name; it is still potentially present for the psychopath.