fix: Normalize Dask timestamps to UTC without a row-wise apply

[Daksha1611]

What this PR does / why we need it:

_normalize_timestamp made timestamp columns tz-aware with a row-wise apply that
declared meta=(timestamp_field, "datetime64[ns, UTC]"). meta only declares the
dtype — the partition gets whatever the lambda returns. With zero rows the lambda never
runs, so the computed column stayed tz-naive while meta claimed UTC, and the tz-aware
comparison in _filter_ttl then raised:

TypeError: Invalid comparison between dtype=datetime64[ns] and DatetimeArray

A zero-row entity_df is a normal case in batch scoring — the upstream query matched
nothing for that run — so get_historical_features crashed instead of returning an
empty result.

While fixing this I found the same mismatch on a second path: a column carrying a
non-UTC timezone also diverged from the declared meta, because the lambda returned any
value that already had tzinfo untouched. That is covered by a test here too.

Both applies are replaced with dd.to_datetime(..., utc=True), which localizes tz-naive
values, converts tz-aware ones, and yields the correct dtype for an empty frame. It is
also vectorized, so it drops a Python-level call per row on non-empty frames.

Which issue(s) this PR fixes:

Fixes #6817

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

New sdk/python/tests/unit/infra/offline_stores/test_dask_empty_entity_df.py covers
normalization and TTL filtering at 0 and 1 rows, plus the non-UTC case. Three of its
five assertions fail on master and pass here; the two 1-row cases pass both ways,
pinning the non-empty behaviour as unchanged.

Verified end to end as well: get_historical_features(...).to_df() against a file
offline store with an otherwise identical entity frame raised TypeError at 0 rows
before this change, and now returns an empty frame with the expected columns while the
1-row case is unchanged.

sdk/python/tests/unit/infra/offline_stores plus test_unit_feature_store.py: 235
passed. The 20 errors in that run are MongoDB testcontainers failing to start locally
and are present on master too.

ruff check, ruff format --check and mypy are clean on both changed files.

Misc

Point-in-time correctness is unaffected — I re-ran the filter_by_created_timestamp
scenario (a row backfilled after the entity timestamp) and it still excludes the late
row with the flag and includes it without.