Don't fail if there is no serializer for type parameters of contextual serializer

Return null instead.
Such behaviour is needed to support cachedChildSerializers logic.
Since this field creator doesn't provide genericGetter (because it's
static),
type param serializer can't be retrieved, and the whole contextual
serializer shouldn't be cached.

#KT-58067 Fixed

(cherry picked from commit fa8f38c)

Author: sandwwraith

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/BaseIrGenerator.kt

@@ -497,7 +497,7 @@ abstract class BaseIrGenerator(private val currentClass: IrClass, final override
                                         compilerContext,
                                         it
                                     )
-                                    instantiate(argSer, it)!!
+                                    instantiate(argSer, it) ?: return null
                                 })
                         )
                     }

plugins/kotlinx-serialization/testData/boxIr/contextualWithTypeParameters.kt

@@ -0,0 +1,44 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.modules.*
+import kotlinx.serialization.encoding.*
+
+class SomeData<T>(val t: T)
+
+@Serializable
+class PagedData<T>(
+    @Contextual val someData: SomeData<T>,
+)
+
+class SomeDataSerializer<T>(val tSer: KSerializer<T>) : KSerializer<SomeData<T>> {
+    override val descriptor: SerialDescriptor = buildClassSerialDescriptor("SomeData")
+
+    override fun serialize(encoder: Encoder, value: SomeData<T>) {
+        encoder as JsonEncoder
+        val data = encoder.json.encodeToJsonElement(tSer, value.t)
+        val obj = buildJsonObject {
+            put("innerType", tSer.descriptor.serialName)
+            put("data", data)
+        }
+        encoder.encodeJsonElement(obj)
+    }
+
+    override fun deserialize(decoder: Decoder): SomeData<T> {
+        TODO("Not yet implemented")
+    }
+}
+
+fun box(): String {
+    val module = SerializersModule {
+        contextual(SomeData::class) { args -> SomeDataSerializer(args[0]) }
+    }
+    val json = Json { serializersModule = module }
+    val input = PagedData<String>(SomeData("foo_bar"))
+    val enc = json.encodeToString(input)
+    return if (enc != """{"someData":{"innerType":"kotlin.String","data":"foo_bar"}}""") enc else "OK"
+}