Handling null correctly when wrapping function in SAM wrapper

compiler/testData/codegen/boxWithJava/samAdapters/nonLiteralNull.java

@@ -0,0 +1,5 @@
+class JavaClass {
+    public static String run(Runnable r) {
+        return r == null ? "OK" : "FAIL";
+    }
+}

compiler/testData/codegen/boxWithJava/samAdapters/nonLiteralNull.kt

@@ -0,0 +1,4 @@
+fun box(): String {
+    val f: (() -> Unit)? = null
+    return JavaClass.run(f)!!
+}