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/testData/boxIr/externalSerialierJava.kt

@@ -25,4 +25,4 @@ object URLSerializer : KSerializer<URL> {
 fun box(): String {
     if (URLSerializer.descriptor.toString() != "PrimitiveDescriptor(java.net.URL)") return URLSerializer.descriptor.toString()
     return "OK"
-}
+}