DatetimeEncoder(resolution=None, periodic_encoding='circular') raises UnboundLocalError #2239

Describe the bug

DatetimeEncoder(resolution=None, periodic_encoding="circular") raises UnboundLocalError instead of transforming, even though resolution=None and periodic_encoding="circular" are each individually documented and accepted by the estimator.

Steps/Code to Reproduce

import pandas as pd
from skrub import DatetimeEncoder

s = pd.to_datetime(pd.Series(["2024-04-14", "2024-05-15"], name="birthday"))
DatetimeEncoder(resolution=None, periodic_encoding="circular").fit_transform(s)

Expected Results

Either a transformed frame with just the features documented for resolution=None (total_seconds, plus weekday/day_of_year if requested), since no calendar levels are extracted for periodic_encoding to apply to, or a clear ValueError if this combination is meant to be disallowed.

Actual Results

Traceback (most recent call last):
  File ".../skrub/_datetime_encoder.py", line 401, in fit_transform
    encoding_levels = list(_DEFAULT_ENCODING_PERIODS.keys())[0:idx_level]
                                                               ^^^^^^^^^
UnboundLocalError: cannot access local variable 'idx_level' where it is not associated with a value

Raised from fit_transform at skrub/_datetime_encoder.py:401, where encoding_levels = list(_DEFAULT_ENCODING_PERIODS.keys())[0:idx_level] reads idx_level, a variable only assigned in the else branch of if self.resolution is None. periodic_encoding="spline" fails the same way.

I have a one-line fix and regression tests ready locally (idx_level = 0 when resolution is None, since no calendar level is extracted, so none can be periodic-encoded). Before opening a PR I'd like maintainer input on one design point: with add_weekday=False, that fix makes resolution=None + periodic_encoding a silent no-op (no periodic features are produced) rather than raising — the same as resolution="year" already does today. Happy to send that PR, or switch to a ValueError/warning instead, let me know which you'd prefer and I'll open it against this issue.

Versions

skrub 0.11.dev0 (current main, c08c674)