Ignore validation on optional arguments

[aoudiamoncef]

Hi @bbakerman, @andimarek,

    findCitiesByCityCode(
        cityCode: String! @NotEmpty @NotBlank @Size(min : 5, max : 5)
        postCode: String @NotEmpty @NotBlank @Size(min : 5, max : 5) # always triggered even if the argument doesn't supplied.
		...
    ): CityPage

I think that adding a flag to directive in order to tell constraints runner to ignore optional validations if argument doesn't exist.

I'll try to add the behavior a PR. Please don't hesitate to give me feedbacks.

Thank you

[enriquedacostacambio]

i believe validation should be completely skipped for all rules if the value is null, given that graphql has nullability checks embedded in the type system.

[aoudiamoncef]

@setchy, what do you think about this PR ?