Make validation message non_null in messages list

[LiboShen]

https://github.com/mirego/absinthe_error_payload/blob/691194ea3360e6e38c8278d5c51f9f9546cd0e5b/lib/absinthe_error_payload/payload.ex#L125

A better schema seems should be like this:

field(:messages, list_of(non_null:validation_message))

Any reason allowing nullable validation messages in the list?