Fix triggerer errors after Airflow 2 to 3 migration

[ephraimbuddy]

When upgrading from Airflow 2, existing deferred triggers can reference TaskInstances without a dag_version_id and DagRuns with conf=None. This caused errors when the triggerer tried to start those triggers and when workers consumed ti_run responses.

This change:

1. Skips starting triggers whose TaskInstance lacks dag_version_id, logging a warning instead of erroring
2. Coerces DagRun.conf from None to {} in the ti_run response for compatibility with Airflow 2-era data
3. Adds unit tests covering both behaviors

This prevents triggerer crashes and makes deferred tasks resume reliably after migration.

closes: #55713

How to test:

set a fernet key e.g:
export AIRFLOW__CORE__FERNET_KEY='8janSoQD86ALy_tnJjR-hcxNweHnUxhfDV61TBntr_4='
in both init.sh and environment_variables.env in airflow source.
AF2 uses init.sh for breeze but environment_variables.env is used in AF3.

Switch to airflow 2: git switch apache/v2-11-stable
Add this dag with the target time adjusted to maybe 30mins in the future:

from datetime import datetime
from airflow import DAG

from airflow.sensors.date_time import DateTimeSensorAsync


with (DAG("async_trigger_sleep", start_date=datetime(2025,9,17),tags=["async_migration"])
      ) as dag:
    DateTimeSensorAsync(
        task_id="wait_for_start",
        target_time=datetime(2025, 9, 19, 11, 26),

    )

start breeze breeze start-airflow --backend postgres --executor CeleryExecutor
Trigger the dag above and stop breeze once the task is deferred.
Switch to Airflow 3: git switch main
Start breeze: breeze start-airflow --backend postgres --executor CeleryExecutor

See that when the target time reach, the dag would be successful

[pdellarciprete]

We are having exactly the same issue, and it is causing the crash of the scheduler. Any plan to release it soon?
Is there any workaround to fix the values on the databases?