hide Node version in prompt when nvm is installed and system node version is beeing used. (#314)

bullet-train.zsh-theme

@@ -589,7 +589,7 @@ prompt_nvm() {
   local nvm_prompt
   if type nvm >/dev/null 2>&1; then
     nvm_prompt=$(nvm current 2>/dev/null)
-    [[ "${nvm_prompt}x" == "x" ]] && return
+    [[ "${nvm_prompt}x" == "x" || "${nvm_prompt}" == "system" ]] && return
   elif type node >/dev/null 2>&1; then
     nvm_prompt="$(node --version)"
   else