Fix bug in previous-successful-dagrun Execution API endpoint

[kaxil]

closes #47612

The bug was as mentioned in #48503 where the logical_date of the currrent TI was None.

We should return an empty PrevSuccessfulDagRunResponse() but because we weren't checking for it, the API server failed with:

airflow-core/tests/unit/api_fastapi/execution_api/versions/head/test_task_instances.py:1160: in test_ti_with_none_as_logical_date
    response = client.get(f"/execution/task-instances/{ti.id}/previous-successful-dagrun")
/usr/local/lib/python3.10/site-packages/starlette/testclient.py:465: in get
    return super().get(
...
...
/usr/local/lib/python3.10/site-packages/cadwyn/schema_generation.py:504: in __call__
    return self._original_callable(*args, **kwargs)
airflow-core/src/airflow/api_fastapi/execution_api/routes/task_instances.py:555: in get_previous_successful_dagrun
    DR.logical_date < task_instance.logical_date,
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/operators.py:368: in __lt__
    return self.operate(lt, other)
/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/attributes.py:322: in operate
    return op(self.comparator, *other, **kwargs)
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/operators.py:368: in __lt__
    return self.operate(lt, other)
/usr/local/lib/python3.10/site-packages/sqlalchemy/orm/properties.py:426: in operate
    return op(self.__clause_element__(), *other, **kwargs)
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/operators.py:368: in __lt__
    return self.operate(lt, other)
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/elements.py:873: in operate
    return op(self.comparator, *other, **kwargs)
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/operators.py:368: in __lt__
    return self.operate(lt, other)
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/type_api.py:1379: in operate
    return super(TypeDecorator.Comparator, self).operate(
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/type_api.py:77: in operate
    return o[0](self.expr, op, *(other + o[1:]), **kwargs)
/usr/local/lib/python3.10/site-packages/sqlalchemy/sql/default_comparator.py:95: in _boolean_compare
    raise exc.ArgumentError(
E   sqlalchemy.exc.ArgumentError: Only '=', '!=', 'is_()', 'is_not()', 'is_distinct_from()', 'is_not_distinct_from()' operators can be used with None/True/False

---

^ 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.

[tirkarthi]

This should close #47612 which is the relevant bug report