[12.x] Improved manager typehints

[daniser]

Looking at this code, getDefaultDriver definitely can return null:

framework/src/Illuminate/Support/Manager.php

Lines 67 to 73 in 7b5e185

	$driver = $driver ?: $this->getDefaultDriver();
	if (is_null($driver)) {
	throw new InvalidArgumentException(sprintf(
	'Unable to resolve NULL driver for [%s].', static::class
	));
	}

This PR adjusts typehint on abstract method in Manager class itself, and also in SessionManager because it has nullable default config value.