Check for deprecation warning

[Baspa]

I was just trying to parse mjml to html but I got the following error message:

In Mjml.php line 143:

array_key_exists(): Argument #2 ($array) must be of type array, null given

Then I checked the $resultString in the mjml.php and it seems I received the following data when I dump the $resultString:

MJML v3 syntax detected, migrating to MJML v4 syntax. Use mjml -m to get the migrated MJML. {"mjmlError":"TypeError: Cannot read properties of undefined (reading 'tagName')"}

This causes the code not being able to parse the result to json. I added a check to see if the resultString contains the deprecation warning. This code can probably be improved but it works for now. Please let me know if I can do anything to improve this PR. I can try to add tests for example.

[freekmurze]

Thanks! Could you indeed add a test for this?

[Baspa]

@freekmurze I've added a test which uses both valid and deprecated mjml. Please let me know if this is sufficient or if more tests and/or code changes are needed.

[freekmurze]

Thanks!