Fixed check if argument exists.
Fixed check if argument exists.

Forgot to use typeof() here.
sojimaxi/texgen.js,mcanthony/texgen.js,mrdoob/texgen.js,mcanthony/texgen.js,sojimaxi/texgen.js,mrdoob/texgen.js,mrdoob/texgen.js,mcanthony/texgen.js,sojimaxi/texgen.js