Fix creating FirSerializableProperties for external classes.

External classes do not have primary constructor, so we need to use 0 as
value parameter count for them to be externally serializable when possible.

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

@@ -0,0 +1,21 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+// SKIP_TXT
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import java.util.*
+
+
+@Serializer(forClass = Date::class)
+object DateSerializer : KSerializer<Date> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("java.util.Date", PrimitiveKind.STRING)
+
+    override fun serialize(encoder: Encoder, value: Date) {
+        TODO()
+    }
+
+    override fun deserialize(decoder: Decoder): Date {
+        TODO()
+    }
+}