Accept None in the migrations_folder_path argument

[sinisaos]

Related to #1161

[dantownsend]

OK, I see the value in this. What happens if we try creating a migration in an app with no migrations - does it raise an error or do nothing?

[sinisaos]

@dantownsend For now UnboundLocalError: cannot access local variable 'path' where it is not associated with a value. If raising ValueError will be better? Something like this

# apps/migrations/commands/new.py
if app_config.resolved_migrations_folder_path is None:
    raise ValueError("Migrations disabled in this app!")

path = os.path.join(
    app_config.resolved_migrations_folder_path, f"{filename}.py"
)