There is a russian joke the optimist is that who says "Ждать, пока чайник закипит" and pessimist is, in turn, a person who'd prefer to say "Ждать, пока чайник не закипит".
Those two form are opposite grammatically and are identical in this context. For example, we can also say "пока стану миллионером" and "пока не стану миллионером".
My question is - are both variants indeed equally accepted, how this phenomenon of loss of negation is called? Is it typical for Russian, or for other Slavic languages as well?