optimize Runtime.default_paths slightly

runtime/lib/runtime.rb

@@ -33,10 +33,13 @@ module Runtime
     #
     # @return [Array] Default runtime paths.
     def default_paths
-      [
-        File.expand_path("~/.jes"),
-        File.expand_path("../../runtime", $0),
-      ]
+      paths = []
+      user_path = File.expand_path("~/.jes")
+      if File.directory?(user_path)
+        paths << user_path
+      end
+      paths << File.expand_path("../../runtime", $0)
+      paths
     end
 
     # Find a runtime file within a single runtime path.