Description: The zero polynomial is homogeneous. Under df-mhp , it has any
(nonnegative integer) degree which loosely corresponds to the value
"undefined". The values -oo and 0 are also used in Metamath (by
df-mdeg and df-dgr respectively) and the literature:
https://math.stackexchange.com/a/1796314/593843 . (Contributed by SN, 12-Sep-2023)