Make the valid_migration function optional

In practice I find it is rarely used, hence it is now optional to save
writing an empty function.

docs/how_to_guides/migrations.rst

@@ -5,8 +5,8 @@ Migrations
 
 Quart-DB considers migrations to be linear and forward only, as such
 migrations are numbered from 0, to 1, onwards. Each migration is a
-python file containing two functions, the first ``migrate`` has the
-following signature,
+python file containing a ``migrate`` function with the following
+signature,
 
 .. code-block:: python
 
@@ -16,16 +16,17 @@ following signature,
 ``migrate`` should run whatever queries are required for the
 migration.
 
-The second ``valid_migration`` has the following signature,
+The file can also contain an optional ``valid_migration`` function
+with the following signature,
 
 .. code-block:: python
 
     async def valid_migration(connection: quart_db.Connection) -> bool:
         ...
 
 ``valid_migration`` should check that the migration was successful and
-the data in the database is as expected. It can just ``return True``
-if this is something you'd prefer to skip.
+the data in the database is as expected. It can be omitted if this is
+something you'd prefer to skip.
 
 Transactions
 ------------

src/quart_db/_migration.py

@@ -75,7 +75,7 @@ async def _run_migration(connection: ConnectionABC, migration: int, migrations_p
     module = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(module)
     await module.migrate(connection)
-    valid = await module.valid_migration(connection)
+    valid = not hasattr(module, "valid_migration") or await module.valid_migration(connection)
     if not valid:
         raise MigrationFailedError(f"Migration {migration} is not valid")
 

tests/migrations/0.py

@@ -21,7 +21,3 @@ async def migrate(connection: Connection) -> None:
                    options OPTIONS_T
             )"""
         )
-
-
-async def valid_migration(connection: Connection) -> bool:
-    return True