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/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/services/FirSerializablePropertiesProvider.kt

@@ -89,7 +89,7 @@ class FirSerializablePropertiesProvider(session: FirSession) : FirExtensionSessi
             .let { restoreCorrectOrderFromClassProtoExtension(classSymbol, it) }
 
         val isExternallySerializable = classSymbol.isEnumClass ||
-                primaryConstructorProperties.size == classSymbol.primaryConstructorSymbol()?.valueParameterSymbols?.size
+                primaryConstructorProperties.size == (classSymbol.primaryConstructorSymbol()?.valueParameterSymbols?.size ?: 0)
 
         val (serializableConstructorProperties, serializableStandaloneProperties) = serializableProperties.partition { it.propertySymbol in primaryConstructorProperties }
         return FirSerializableProperties(

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()
+    }
+}

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirDiagnosticTestGenerated.java

@@ -44,6 +44,12 @@ public class SerializationFirDiagnosticTestGenerated extends AbstractSerializati
             runTest("plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.kt");
         }
 
+        @Test
+        @TestMetadata("externalSerialierJava.kt")
+        public void testExternalSerialierJava() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/diagnostics/externalSerialierJava.kt");
+        }
+
         @Test
         @TestMetadata("ExternalSerializers.kt")
         public void testExternalSerializers() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationPluginDiagnosticTestGenerated.java

@@ -42,6 +42,12 @@ public class SerializationPluginDiagnosticTestGenerated extends AbstractSerializ
         runTest("plugins/kotlinx-serialization/testData/diagnostics/EnumDuplicateSerialName.kt");
     }
 
+    @Test
+    @TestMetadata("externalSerialierJava.kt")
+    public void testExternalSerialierJava() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/diagnostics/externalSerialierJava.kt");
+    }
+
     @Test
     @TestMetadata("ExternalSerializers.kt")
     public void testExternalSerializers() throws Exception {