Write a SpacetimeDB backend module in Rust that defines a table with optional (nullable) fields and a reducer that inserts rows with and without optional values.

TABLES
- player
  - Struct: Player
  - Fields:
    - id: u64 (primary key, auto_inc)
    - name: String
    - nickname: Option<String>
    - high_score: Option<u32>

REDUCERS
- create_player(ctx, name: String, nickname: Option<String>, high_score: Option<u32>)
  - Insert a Player row, passing through the optional fields as-is
  - Use id: 0 for auto_inc
