Do not create cacheableChildSerializers unless it is necessary

Creation of this property in the SerializerIrGenerator.<init> can lead to
a 'Serializer not found' internal error when generator is applied to a fully-customized external serializer.
In that case, generator is still created, but none of the generateSave/Load functions are called,
so cacheableChildSerializers(Property) is not necessary.

#KT-57730 Fixed
Fixes https://github.com/Kotlin/kotlinx.serialization/issues/2260

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

@@ -81,6 +81,12 @@ public class SerializationFirLightTreeBlackBoxTestGenerated extends AbstractSeri
             runTest("plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt");
         }
 
+        @Test
+        @TestMetadata("externalSerializerForClassWithNonSerializableType.kt")
+        public void testExternalSerializerForClassWithNonSerializableType() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/externalSerializerForClassWithNonSerializableType.kt");
+        }
+
         @Test
         @TestMetadata("generatedClassifiersViaLibraryDependency.kt")
         public void testGeneratedClassifiersViaLibraryDependency() throws Exception {

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

@@ -79,6 +79,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt");
     }
 
+    @Test
+    @TestMetadata("externalSerializerForClassWithNonSerializableType.kt")
+    public void testExternalSerializerForClassWithNonSerializableType() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/externalSerializerForClassWithNonSerializableType.kt");
+    }
+
     @Test
     @TestMetadata("generatedClassifiersViaLibraryDependency.kt")
     public void testGeneratedClassifiersViaLibraryDependency() throws Exception {