Do not unbox nullable Result, since before usage it is coerced

#KT-46505

compiler/testData/codegen/box/inlineClasses/callableReferences/let/int.kt

@@ -6,11 +6,11 @@ object Foo {
     }
 
     fun bar(value: Value?) {
-        res = value?.value!!
+        res = value?.value
     }
 }
 
-var res = 0
+var res: Int? = 0
 
 fun box(): String {
     Value(42).let(Foo::foo)
@@ -19,6 +19,10 @@ fun box(): String {
 
     Value(42).let(Foo::bar)
     if (res != 42) return "FAIL 2 $res"
+    res = 0
+
+    null.let(Foo::bar)
+    if (res != null) return "FAIL 3: $res"
 
     return "OK"
 }