added option to generate empty migration file

aerich/__init__.py

@@ -123,8 +123,8 @@ class Command:
         inspect = cls(connection, tables)
         return await inspect.inspect()
 
-    async def migrate(self, name: str = "update"):
-        return await Migrate.migrate(name)
+    async def migrate(self, name: str = "update", empty: bool = False) -> str:
+        return await Migrate.migrate(name, empty)
 
     async def init_db(self, safe: bool):
         location = self.location

aerich/migrate.py

@@ -130,12 +130,16 @@ class Migrate:
         return version
 
     @classmethod
-    async def migrate(cls, name) -> str:
+    async def migrate(cls, name: str, empty: bool) -> str:
         """
         diff old models and new models to generate diff content
-        :param name:
+        :param name: str name for migration
+        :param empty: bool if True generates empty migration
         :return:
         """
+        if empty:
+            return await cls._generate_diff_py(name)
+
         new_version_content = get_models_describe(cls.app)
         cls.diff_models(cls._last_version_content, new_version_content)
         cls.diff_models(new_version_content, cls._last_version_content, False)