Description

Two correctness bugs in DataPortal (src/zipline/data/data_portal.py):

1. _first_trading_minute set to (None, None) instead of None

In DataPortal.__init__(), when _first_trading_day is None, _first_trading_minute is set to (None, None) (a tuple) instead of None. Since session_first_minute() returns a single pd.Timestamp, the else branch should return None to match the expected type.

This causes TypeError in _handle_minute_history_out_of_bounds (comparing a Timestamp with a tuple) and _get_history_minute_window (comparing Timestamp < tuple).

2. _get_daily_spot_value passes "minute" instead of "daily"

In _get_daily_spot_value(), when forward-filling the "price" field and finding a value on a previous day, the code calls get_adjusted_value(..., "minute", ...) but this is the daily code path and should pass "daily".

While this doesn't currently cause incorrect results (the spot_value parameter bypasses the frequency-dependent path), it violates the semantic contract and could cause bugs if get_adjusted_value is refactored to use data_frequency in the adjustment path.