[9.x] Add assertDatabaseEmpty helper

[christophrumpel]

This PR adds a new testing helper assertDatabaseEmpty which checks if a specific table has no entires.

Why do we need it?

When testing against the database you often also need to check if a table is empty before or after a specific action:

    // Assert
    $this->assertDatabaseCount(MyModel::class, 0);

    // Act
    // Call a specific action or request

    // Assert
    $this->assertDatabaseCount(MyModel::class, 1);

This new testing helper is a bit shorter to write, descriptive, and better to read:

    // Assert
    $this->assertDatabaseEmpty(MyModel::class);

    // Act
    // Call a specific action or request

    // Assert
    $this->assertDatabaseCount(MyModel::class, 1);