fix directives return nil handling

[vvakame]

Describe your PR and link to any relevant issues.
related #799 and #819 🤔 ??

.TypeReference.IsNilable provides Go side type information.
directives.gotpl, args.gotpl and input.gotpl use it.
so, I think usage is split to 2 ways.

1. The value bind to Go field (or variable). ( args.gotpl and input.gotpl )
2. The value map to graphql.Marshaler. ( directives.gotpl )

I think #2 pattern should use .TypeReference.GQL.NonNull instead of .TypeReference.IsNilable.

My change returns nil, and if it is invalid. caught by here →

gqlgen/codegen/field.gotpl

Lines 79 to 86 in 5c644a6

	if resTmp == nil {
	{{- if $field.TypeReference.GQL.NonNull }}
	if !ec.HasError(rctx) {
	ec.Errorf(ctx, "must not be null")
	}
	{{- end }}
	return graphql.Null
	}

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[vektah]

🤔
Right, because directives always use interface{} they can always be nil, and we don't care what the datatype is. Unlike everywhere else where we cant compare a string to nil without getting a compile-time error.

👍

[coveralls]

Coverage remained the same at 61.817% when pulling f33e09e on fix-directive-nil into 8590ede on master.