[Fix] Add default empty value for NVM_NO_ALIAS variable

[sehrope]

Adds a default value for NVM_NO_ALIAS so that nvm ls does not error out when run in a bash nounset/-u (no unset vars) environment.

Reproduce error via:

$ set -u ; nvm ls 12
bash: NVM_NO_ALIAS: unbound variable

[ljharb]

Thanks! Can we add a test that would catch this?

[ljharb]

(Note that when I run set -u; nvm ls 0.13, for example, I don't get this error)

[sehrope]

Added a test for enabling nounset and checking for any stderr in a couple situations of nvm ls .... It fails when applied on it's own: https://travis-ci.org/sehrope/nvm/jobs/532527831#L390

With the fix applied, the test passes. 🎉

[ljharb]

Thanks!

Still confused why this can't repro on my Mac; maybe it's got an outdated or broken bash.