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/BlackBoxCodegenTestGenerated.java

@@ -15372,6 +15372,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/inlineClasses/toStringCallingPrivateFun.kt");
         }
 
+        @TestMetadata("toStringOfUnboxedNullable.kt")
+        public void testToStringOfUnboxedNullable() throws Exception {
+            runTest("compiler/testData/codegen/box/inlineClasses/toStringOfUnboxedNullable.kt");
+        }
+
         @TestMetadata("typeChecksForInlineClasses.kt")
         public void testTypeChecksForInlineClasses() throws Exception {
             runTest("compiler/testData/codegen/box/inlineClasses/typeChecksForInlineClasses.kt");