Adding type checking assert to cron.get_next

[amoghrajesh]

Croniter released new version:

564c564
< types-croniter==5.0.1.20250322
---
> types-croniter==6.0.0.20250411

Due to this the cron.get_prev can return float as well causing mypy to complain:

airflow-core/src/airflow/timetables/_cron.py:112: error: Incompatible return
value type (got "None", expected "DateTime")  [return-value]
                return convert_to_utc(make_aware(scheduled, self._timezone...
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
airflow-core/src/airflow/timetables/_cron.py:112: error: No overload variant of
"make_aware" matches argument types "float", "Union[Timezone, FixedTimezone]" 
[call-overload]
                return convert_to_utc(make_aware(scheduled, self._timezone...
                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
airflow-core/src/airflow/timetables/_cron.py:112: note: Possible overload variants:
airflow-core/src/airflow/timetables/_cron.py:112: note:     def make_aware(value: None, timezone: Optional[tzinfo] = ...) -> None
airflow-core/src/airflow/timetables/_cron.py:112: note:     def make_aware(value: DateTime, timezone: Optional[tzinfo] = ...) -> DateTime
airflow-core/src/airflow/timetables/_cron.py:112: note:     def make_aware(value: datetime, timezone: Optional[tzinfo] = ...) -> datetime
Found 2 errors in 1 file (checked 971 source files)
Error 1 returned
You are running mypy with the folders selected. If you want to reproduce it locally, you need to run the following command:

pre-commit run --hook-stage manual mypy-<folder> --all-files

Added a type checking for it so that its happy 🗡️

Example: https://github.com/apache/airflow/actions/runs/14404400215/job/40397831030

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.