Update go dependencies (#4322)

vendor/github.com/go-openapi/spec/schema.go

@@ -163,6 +163,7 @@ type SchemaProps struct {
 	Schema               SchemaURL         `json:"-"`
 	Description          string            `json:"description,omitempty"`
 	Type                 StringOrArray     `json:"type,omitempty"`
+	Nullable             bool              `json:"nullable,omitempty"`
 	Format               string            `json:"format,omitempty"`
 	Title                string            `json:"title,omitempty"`
 	Default              interface{}       `json:"default,omitempty"`
@@ -302,6 +303,12 @@ func (s *Schema) AddType(tpe, format string) *Schema {
 	return s
 }
 
+// AsNullable flags this schema as nullable.
+func (s *Schema) AsNullable() *Schema {
+	s.Nullable = true
+	return s
+}
+
 // CollectionOf a fluent builder method for an array parameter
 func (s *Schema) CollectionOf(items Schema) *Schema {
 	s.Type = []string{jsonArray}