Hi, By the null consonant, do you mean ㅇ(이응)? In fact, I never thought of this issue before, so I did right now. According to the Korean wikipedia, ㅇ used as an initial sound does not have any sound (or sound value?). (http://ko.wikipedia.org/wiki/%E3%85%87) In addition the shape of ㅇ was meant to look similar to the throat. Here is my guess: When you pronounce the vowels, you don't usually use any other part of mouth, tongue, and nose except for the throat.

hmm, it's like in arabic they use zero like five:D :P the korean zero is not insignificant...they are many words including it...like...ur name(alsaidi) ...it's 알싸이디 in korean... do u see how many zeroes are in ur name?:P