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

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

@@ -57,6 +57,12 @@ public class SerializationFirLightTreeBlackBoxTestGenerated extends AbstractSeri
             runTest("plugins/kotlinx-serialization/testData/boxIr/contextualFallback.kt");
         }
 
+        @Test
+        @TestMetadata("contextualWithTypeParameters.kt")
+        public void testContextualWithTypeParameters() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/contextualWithTypeParameters.kt");
+        }
+
         @Test
         @TestMetadata("delegatedInterface.kt")
         public void testDelegatedInterface() throws Exception {

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

@@ -55,6 +55,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/contextualFallback.kt");
     }
 
+    @Test
+    @TestMetadata("contextualWithTypeParameters.kt")
+    public void testContextualWithTypeParameters() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/contextualWithTypeParameters.kt");
+    }
+
     @Test
     @TestMetadata("delegatedInterface.kt")
     public void testDelegatedInterface() throws Exception {