Update trigger kwargs migration to specify existing_nullable (#39361)

Without the existing_nullable set during alter_column, MySQL will set it to Null.
See https://alembic.sqlalchemy.org/en/latest/ops.html#alembic.operations.Operations.alter_column.params.existing_nullable

(cherry picked from commit 1d504ca)

Author: ephraimbuddy

airflow/migrations/versions/0140_2_9_0_update_trigger_kwargs_type.py

@@ -50,7 +50,7 @@ def get_session() -> sa.orm.Session:
 def upgrade():
     """Update trigger kwargs type to string and encrypt"""
     with op.batch_alter_table("trigger") as batch_op:
-        batch_op.alter_column("kwargs", type_=sa.Text(), )
+        batch_op.alter_column("kwargs", type_=sa.Text(), existing_nullable=False)
 
     if not context.is_offline_mode():
         session = get_session()

docs/apache-airflow/img/airflow_erd.sha256

@@ -1 +1 @@
-818cbc5af55096456b556d62e73eb5d797001969a04256f4a99e097fe189855d
+4a8aed085996d893909588dabe45906213629a545bbce7b551189c85121f88bd