[Fix] `set -u`: ensure `NVM_USE_OUTPUT` is always set.

Fixes #1671.

nvm.sh

@@ -508,7 +508,7 @@ nvm_remote_versions() {
   local NVM_LS_REMOTE_IOJS_EXIT_CODE
   NVM_LS_REMOTE_IOJS_EXIT_CODE=0
   local NVM_LS_REMOTE_IOJS_OUTPUT
-  NVM_LS_REMOTE_IOJS_OUTPUT=
+  NVM_LS_REMOTE_IOJS_OUTPUT=''
   if [ -z "${NVM_LTS-}" ] && ( \
     [ -z "${NVM_FLAVOR-}" ] || [ "${NVM_FLAVOR-}" = "${NVM_IOJS_PREFIX}" ] \
   ); then
@@ -2902,6 +2902,7 @@ nvm() {
         command rm -f "$NVM_DIR/current" && ln -s "$NVM_VERSION_DIR" "$NVM_DIR/current"
       fi
       local NVM_USE_OUTPUT
+      NVM_USE_OUTPUT=''
       if [ $NVM_USE_SILENT -ne 1 ]; then
         if nvm_is_iojs_version "$VERSION"; then
           NVM_USE_OUTPUT="Now using io.js $(nvm_strip_iojs_prefix "$VERSION")$(nvm_print_npm_version)"

test/fast/Running "nvm use system" should work as expected

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-set -ex
+set -eux
 
 cleanup() {
   unset -f nvm_has_system_node nvm_print_npm_version