fix: guard get_next/get_previous against unset _more

[CedricConday]

Description

ResultSet.get_next() / get_previous() guard with not hasattr(self, "_more"), but _more is a declared Pydantic private attribute with a default of None:

class ResultSet(BaseModel):
    _more: Optional[Callable] = None

Because the attribute is declared, hasattr(self, "_more") is always True (it resolves to the default None), so the guard never short-circuits. When _more has not been set — e.g. a ResultSet that wasn't produced by the @returns decorator — the methods fall through to self._more(**params) and raise:

TypeError: 'NoneType' object is not callable

This changes the guard to self._more is None, so get_next() / get_previous() return None as intended when there is no callable to page with. The normal decorator-driven flow (where _more is a functools.partial) is unchanged.

Testing

Added test_resultset_without_more_returns_none, which constructs a ResultSet with next/previous set but no _more and asserts both accessors return None. Before the change this test raised TypeError; the existing test_resultset (decorator-driven paging) continues to pass.