Don't set node-info if version is 'none'

This is done to make it easier to not show node-info in themes
when no node version is currently in use. This is consistent with
how the Python module's python-info works.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>

modules/node/functions/node-info

@@ -17,7 +17,7 @@ if (( $+functions[nvm_version] )); then
   version="${$(nvm_version)#v}"
 fi
 
-if [[ -n "$version" ]]; then
+if [[ "$version" == (none|) ]]; then
   zstyle -s ':prezto:module:node:info:version' format 'version_format'
   zformat -f version_formatted "$version_format" "v:$version"
   node_info[version]="$version_formatted"