The database backend specific checks could be bypassed when using multiple databases with one empty default database.

[Shipeng Feng]

Here is the settings:

DATABASES = {
    'default': {},
    'users': {
        'NAME': 'user_data',
        'ENGINE': 'django.db.backends.mysql',
        'USER': 'mysql_user',
        'PASSWORD': 'superS3cret'
    },
}

Here is the related source code: ​https://github.com/django/django/blob/f97a6123c07de5099fdf8b7d00ef7d20ed354e07/django/db/models/fields/__init__.py#L340

We only run checks for the first db, and the behavior of the first db is undetermined (the first entry of a dictionary),
if the first db is default, then we would only run dummy backend checks, mysql checks are bypassed.