sorry, previously i had intended to ask why is the degree of a zero polynomial not defined?
Consider the zero polynomial as p(x) i.e. p(x) = 0
Now, 0 can be written as 0.xm, where m is an integer.
There is no specific value of m. For an instance, m can be taken as 1 since, 0 = 0.x1
Also, m can be taken as -10 since, 0 = 0.x-10
Since, m has not any specific value so, degree of p(x) i.e. zero polynomial is defined.