Add more tests for SAM wrappers

compiler/testData/codegen/box/sam/nullableSam.kt

@@ -0,0 +1,26 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+// FILE: Test.java
+
+public class Test {
+    public static boolean isNull(Runnable r) {
+        if (r == null)
+            return true;
+        r.run();
+        return false;
+    }
+}
+
+// FILE: test.kt
+
+fun nullableFun(fromNull: Boolean): (() -> Unit)? =
+    if (fromNull) null else {{}}
+
+fun box(): String {
+    if (!Test.isNull(nullableFun(true))) return "Fail 1"
+    if (Test.isNull(nullableFun(false))) return "Fail 2"
+    if (!Test.isNull(null)) return "Fail 3"
+    if (Test.isNull {}) return "Fail 4"
+    return "OK"
+}