exception `You are running with multiple databases` with no connections

Describe the bug
I am using the initializer function for my tests but when I try to get_connection, de default one, it actually throws me an error because Tortoise._connections is empty. That is because that function actually empties the dictionary. The thing is that get_connection use that as the source of connections but in reality is current_transaction_map dict.

tortoise.exceptions.ParamsError: You are running with multiple databases, so you should specify connection_name: []

To Reproduce
Steps to reproduce the behavior, preferaby a small code snippet.

• call initializer func
• call get_connection()

Expected behavior
To actually get the connection.

Additional context

This error actually popped me when trying to use in_transaction or atomic functions.

Trace the initializer and get_connection entry points, then compare how Tortoise._connections and current_transaction_map are populated and read. Reproduce the empty-connection case after initializer; done means get_connection() returns the default connection and in_transaction or atomic no longer raises the multiple-databases error.