Check err object to be null

[rehangit]

Our logger ends up falling apart here as the object being serialized is null. Although this can be fixed at source but it should be possible to pass a null object. The serializer should handle the null object gracefully.

pino-std-serializers/lib/err.js

Line 7 in ddb1fb9

type: err.constructor.name,

[mcollina]

Fixed by #15.