[Bug] Setting default_log_name to null breaks logging

[stevebauman]

Describe the bug
The activity_log schema for log_name is nullable(), yet setting default_log_name inside of the configuration breaks logging entirely.

To Reproduce
Set activity_log_name to null, then try logging:

config(['activitylog.default_log_name' => null]);

activity()->log('foo');

Exception

Spatie\Activitylog\ActivityLogger::useLog(): Argument #1 ($logName) must be of type string, null given, called in ../laravel-activitylog/src/helpers.php on line 14

Expected behavior
Since the database table schema supports a nullable log_name, we should be able to create log entries with a null log name instead of an empty string, so that we may use Activity::whereNull('log_name').