Fix pj() function when no project has been specified

plugins/pj/pj.plugin.zsh

@@ -15,6 +15,11 @@ pj () {
         project=$*
     fi
 
+    if [[ -z "$project" ]]; then
+        echo "You have to specify a project name."
+        return
+    fi
+
     for basedir ($PROJECT_PATHS); do
         if [[ -d "$basedir/$project" ]]; then
             $cmd "$basedir/$project"