NVM: Avoid providing completions when nvm is not installed

plugins/nvm/_nvm

@@ -1,6 +1,8 @@
 #compdef nvm
 #autoload
 
+[[ -s ~/.nvm/nvm.sh ]] || return 0
+
 local -a _1st_arguments
 _1st_arguments=(
   'help:show help'