Fix NVM prompt to return if not used

This commit changes nvm prompt setup to return immediately if no node
version is installed/available.
Currently, prompt complains for lack of global node version.
Specifically, if you install nvm (from brew or manually) and do not have
global node installed (or set), then the prompt should not set or show
anything. This commit ensures this by immediately returning if
`nvm current` returns "none".

powerlevel9k.zsh-theme

@@ -560,6 +560,7 @@ prompt_node_version() {
 prompt_nvm() {
   [[ ! $(type nvm) =~ 'nvm is a shell function'* ]] && return
   local node_version=$(nvm current)
+  [[ ${node_version} = "none" ]] && return
   local nvm_default=$(cat $NVM_DIR/alias/default)
   [[ -z "${node_version}" ]] && return
   [[ "$node_version" =~ "$nvm_default" ]] && return