feat: convert None to empty string and validate action parameter type in step() method

[KempinskiB]

No description provided.

[bobbycxy]

Hey Ben, sorry could you share what's the issue? Generally, if no action is provided, the environment returned is still a "str" class.

[borgr]

Recommend merge. Two independent changes, both safe:

1. Assert-message fix (genuine correctness): the check is assert 6 <= num_players <= 15 but the message read "between 5 and 15". A user passing num_players=5 would get an error message telling them 5 is allowed. This is unambiguously worth fixing on its own.

2. step() input guard (harmless defensive validation): to @bobbycxy's question — you're right that on the normal path action is already a str, so this is a no-op for real gameplay. It only adds a graceful None → "" coercion and a clear TypeError for a non-str/non-None arg, instead of failing later with a more confusing error deeper in _handle_*. Nothing about the str path changes, so there's no behavioral risk either way.

Given (1) is a real fix and (2) is harmless, this is fine to merge. Only nit: there's a trailing-whitespace blank line inside the new guard block — worth stripping before merge, but not a blocker.

[borgr]

Verified. The assert-message correction is unambiguously right — the guard is assert 6 <= num_players <= 15, so the old "between 5 and 15" text was wrong; "between 6 and 15" now matches. Optional is already imported, so the Optional[str] signature is fine.

The None → "" / TypeError handling in step() is reasonable defensive input-validation and doesn't change behavior for valid callers. Two small optional nits if you're tidying before merge: the inserted blank line carries trailing whitespace, and the type-check is arguably more than the surrounding envs do — happy either way.

Core fix LGTM, recommend merge.