[Fix] set -u: Add default empty value for $NVM_NO_ALIAS

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.

test/fast/Listing versions/Running "nvm ls" with nounset should not fail.

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+die () { echo "$@" ; exit 1; }
+
+\. ../../../nvm.sh
+\. ../../common.sh
+
+make_fake_node v0.12.34 || die 'fake v0.12.34 could not be made'
+
+# Enable no unset variable
+set -u
+
+# Try an alias that does not exist
+output=$(nvm ls 99 2>&1 1>/dev/null || true)
+test -z "${output}" || die "1: expected empty; got >${output}"
+
+# Try a version that does not exist
+output=$(nvm ls 0.12.00 2>&1 1>/dev/null || true)
+test -z "${output}" || die "2: expected empty; got >${output}"
+
+# Try a version that does exist
+output=$(nvm ls 0.12.34 2>&1 1>/dev/null || true)
+test -z "${output}" || die "3: expected empty; got >${output}"