If either vector a=0 or b=0, then a.b=0. But the converse need not be true .

if  either vector a=0 or b=0, then a.b=0.  But if a.b = 0 then we cant conclude  that one of vectors a or b is zero

a.b =ab cos θ if θ is 90^o  then cos 90^o = 0 hence a.b will be zero even if both a and b are non zero vectors