fix(types): playerID in playerView can be string or null

[ben196888]

This pull request relates to issue #989.

Recap: the playerID in PlayerView could be null when the user is in spectator mode.
Changes:

• update playerID type to be PlayerID | null
• mention the playerID would be null in secret-state document page

Checklist

• Use a separate branch in your local repo (not main).
• Test coverage is 100% (or you have a story for why it's ok).

[delucis]

Thanks @ben196888!