Returned support for nullable types by not null serializers

Fixes Kotlin/kotlinx.serialization#1265

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt

@@ -125,7 +125,7 @@ fun AbstractSerialGenerator.findTypeSerializerOrContextUnchecked(
     if (kType.isTypeParameter()) return null
     annotations.serializableWith(module)?.let { return it.toClassDescriptor }
     additionalSerializersInScopeOfCurrentFile[kType]?.let { return it }
-
+    if (kType.isMarkedNullable) return findTypeSerializerOrContextUnchecked(module, kType.makeNotNullable())
     if (kType in contextualKClassListInCurrentFile) return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
     return analyzeSpecialSerializers(module, annotations) ?: findTypeSerializer(module, kType)
 }