fix: Input object null fields validation

core/src/main/scala/caliban/validation/Validator.scala

@@ -472,7 +472,7 @@ object Validator {
             IO.when(
               inputField.defaultValue.isEmpty &&
                 inputField.`type`().kind == __TypeKind.NON_NULL &&
-                !fields.contains(inputField.name)
+                fields.get(inputField.name).getOrElse(NullValue) == NullValue
             )(
               failValidation(
                 s"Required field '${inputField.name}' on object '${inputType.name.getOrElse("?")}' was not provided.",

core/src/test/scala/caliban/validation/InputObjectSpec.scala

@@ -0,0 +1,32 @@
+package caliban.validation
+
+import caliban.CalibanError
+import caliban.GraphQL._
+import caliban.RootResolver
+import zio.test.Assertion._
+import zio.test._
+import zio.test.environment.TestEnvironment
+
+object InputObjectSpec extends DefaultRunnableSpec {
+  override def spec: ZSpec[TestEnvironment, Any] =
+    suite("InputObjectSpect")(
+      testM("fails if a non-null field on an input object is null") {
+        val query =
+          """query {
+            |  query(input: {string: null})
+            |}""".stripMargin
+
+        case class TestInputObject(string: String)
+        case class TestInput(input: TestInputObject)
+        case class Query(query: TestInput => String)
+        val gql = graphQL(RootResolver(Query(_.input.string)))
+
+        for {
+          int <- gql.interpreter
+          res <- int.execute(query)
+        } yield assert(res.errors.headOption)(
+          isSome((isSubtype[CalibanError.ValidationError](anything)))
+        )
+      }
+    )
+}