Fixed missing fallback serializer in contextual serializer

Fixes Kotlin/kotlinx.serialization#2158

Merge-request: KT-MR-8338
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -0,0 +1,22 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.encoding.*
+
+@Serializable
+data class Holder<T>(
+    val ok: Boolean,
+    @Contextual
+    val result: T?
+)
+
+fun box(): String {
+    val serializer = serializer<Holder<List<String>>>()
+    val instance = Holder(true, listOf("a", "b"))
+    val encoded = Json.encodeToString(serializer, instance)
+    if (encoded != """{"ok":true,"result":["a","b"]}""") return encoded
+    return "OK"
+}