Checkstyle: don't fail if rule attribute is not present

[hectorj]

Hi!

I'm having an issue with parsing checkstyle files coming from phpstan using this lib, as their errors don't have the source attribute, which makes this lib log an error and ignore the violation.

Would it be possible to consider the source as optional?

I am also opening an issue on phpstan to add the source attribute, as I am not sure which fix would be better.

[hectorj]

phpstan issue for reference : phpstan/phpstan#481

[tomasbjerre]

Yes no problem. I'm releasing it now, will release the plugins later today.

[hectorj]

Thanks a lot!