[Fix] `nvm debug`: use default empty values for potentially unset variables

These variables may be unset, which would cause errors in shells with `set -u` (nounset) enabled.

Bugs introduced in https://github.com/nvm-sh/nvm/commit/b805de80cb8058f09fecb11786d4d2aec0dac35c and https://github.com/nvm-sh/nvm/commit/49e446dac18ce8b1267107849e6be6fe99390543.