JVM_IR: check for null when converting unboxed inline classes to strings

`C?` can be unboxed into `T?` if if wraps a reference type `T!!`, but in
this case `null` is not a valid value to pass to `toString-impl`.

 #KT-42005 Fixed

compiler/tests/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -14669,6 +14669,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/inlineClasses/simpleSecondaryConstructor.kt");
         }
 
+        @TestMetadata("toStringOfUnboxedNullable.kt")
+        public void ignoreToStringOfUnboxedNullable() throws Exception {
+            runTest("compiler/testData/codegen/box/inlineClasses/toStringOfUnboxedNullable.kt");
+        }
+
         private void runTest(String testDataFilePath) throws Exception {
             KotlinTestUtils.runTest(this::doTest, TargetBackend.JVM, testDataFilePath);
         }