public void Defensive()
        {
            Assert.Empty(OpenApiSchemaExtensions.GetSchemaReferenceIds(null));
            var schema = new OpenApiSchema {
                AnyOf = null
            };

            Assert.Null(schema.AnyOf);
            Assert.Empty(schema.GetSchemaReferenceIds());
            schema = new() {
                AllOf = null
            };
            Assert.Null(schema.AllOf);
            Assert.Empty(schema.GetSchemaReferenceIds());
            schema = new() {
                OneOf = null
            };
            Assert.Null(schema.OneOf);
            Assert.Empty(schema.GetSchemaReferenceIds());
            schema = new() {
                Properties = null
            };
            Assert.Null(schema.Properties);
            Assert.Empty(schema.GetSchemaReferenceIds());
            Assert.False(OpenApiSchemaExtensions.IsAllOf(null));
            Assert.False(OpenApiSchemaExtensions.IsAnyOf(null));
            Assert.False(OpenApiSchemaExtensions.IsOneOf(null));
            Assert.False(OpenApiSchemaExtensions.IsArray(null));
            Assert.False(OpenApiSchemaExtensions.IsObject(null));
            Assert.False(OpenApiSchemaExtensions.IsReferencedSchema(null));

            Assert.False(new OpenApiSchema {
                Reference = null
            }.IsReferencedSchema());
            Assert.False(new OpenApiSchema {
                Type = null
            }.IsArray());
            Assert.False(new OpenApiSchema {
                Type = null
            }.IsObject());
            Assert.False(new OpenApiSchema {
                AnyOf = null
            }.IsAnyOf());
            Assert.False(new OpenApiSchema {
                AllOf = null
            }.IsAllOf());
            Assert.False(new OpenApiSchema {
                OneOf = null
            }.IsOneOf());
        }

        [Fact]
        public void GetReferenceIdsEmpty()
        {
            var schema = new OpenApiSchema {
            };
            var names  = schema.GetSchemaReferenceIds();

            Assert.Empty(names);
        }