Description: The degree of the zero polynomial is zero. Note: this differs from some
other definitions of the degree of the zero polynomial, such as
-u 1 , -oo or undefined. But it is convenient for us to define it
this way, so that we have dgrcl , dgreq0 and coeid without having
to special-case zero, although plydivalg is a little more complicated
as a result. (Contributed by Mario Carneiro, 22-Jul-2014)