Skip properties from Java classes for which the getter type is unknown

thus avoiding NPE when auto-generating external serializers.

#KT-55681 Fixed

plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt

@@ -0,0 +1,28 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import java.net.URL
+
+
+// @Serializer should do nothing if all methods are overriden
+@Serializer(forClass = URL::class)
+object URLSerializer : KSerializer<URL> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("java.net.URL", PrimitiveKind.STRING)
+
+    override fun serialize(encoder: Encoder, value: URL) {
+        TODO()
+    }
+
+    override fun deserialize(decoder: Decoder): URL {
+        TODO()
+    }
+}
+
+fun box(): String {
+    if (URLSerializer.descriptor.toString() != "PrimitiveDescriptor(java.net.URL)") return URLSerializer.descriptor.toString()
+    return "OK"
+}