Allow games without an End event

[NickCondron]

There's a long-standing bug in Slippi where Game End events are not always written to the Slippi file (see first paragraph in spec for Game End event https://github.com/project-slippi/slippi-wiki/blob/master/SPEC.md#game-end). There are a lot of normal replay files that are only missing the game end event.

Currently, peppi fails to parse these replays into a game considering it a parse error. This behavior is sound considering the spec technically guarantees the last event to be a Game End. However, it is not useful because there are many normal games that fail to be parsed. Slippi-js parses such files without complaint.