Update Spawn_Event.rb

Hi,

I'm using your Spawn Events code for my VX Ace roguelike. One issue I ran into is line 78 of `Spawn_Event.rb`:

`key_id = @events.keys.max + 1`

Since I'm generating my map tiles early in the map life-cycle, there are no events yet; `@events` is an empty hash. In this case, `@events.keys.max` returns nil.

The fix is pretty easy (default to zero instead of nil). This is all that I did in my PR.

Please give me your feedback.