There are many occasions where the last consonant is dropped because it sounds weird and is hard to pronounce two consonants in a row. I had a link to all those rules, but i can't find it now. I'm gonna look for it. =( In this case ᄉ is dropped and ᄇ becomes ᄆ. Just like when you say 합니다 It sounds like 함니다. I'm pretty sure this only happens when ᄇ meets ᄂ. There could be other cases. But i don't think so. =P

No just ignore pronounce ㅅ in there. It is pronouced "eop" If you add to "s", it will sound really funny ㅋㅋㅋㅋ when you say "없다" , just ignore it and pronounce "eop-dda" But remember this, when you face "ㅇ" the next of "없" you should pronouce ㅅ too: "없어" -> 없 어 -> 업 써 = eop - SSer Basically, a consonant word "ㅇ" is dealed with No phonetic value; So "ㅅ" , as a main phonetic value "S", replace the position of "ㅇ".