Add message level when using custom warning handler

[mcg-web]

I need to make silent error_trigger but since it was already refused here I'm going to implement my own handler make it silent. This will fix the issue when using isTypeOf the first entry is null because of the warnOnce that trigger error not silently. Thanks to @murtukov while trying to write some documentation on isTypeOf...

[murtukov]

I can confirm this. When I try to return a list of interfaces and use isTypeOf on each implementing type (instead of single resolveType) the first result in the returned collection is always null.

But adding \GraphQL\Error\Warning::suppress(); to the index.php of my Symfony project seems to fix this problem and I get all results as expected.

[vladar]

This will fix the issue when using isTypeOf the first entry is null because of the warnOnce that trigger error not silently

Why does it return null? I guess Symfony has a custom error handler that throws an exception when there is a warning?

Anyways, the change seems reasonable. Thanks!