Get rid of redundant not-null assertion for parameter

The problem was that in `Function<T>.apply(T)` T is now not-platform,
so when checking if not-null assertion is needed for parameter in SAM,
it's defined by the upper bounds of T that is a platform (Any..Any?),
and while it's definitely not marked as nullable it's still nullable
in a sense that it can contain null as a value.

So the solution is obvious

 #KT-16413 Fixed

compiler/testData/codegen/java8/box/functionAssertion.kt

@@ -0,0 +1,21 @@
+// FILE: F.java
+import java.util.function.Function;
+
+public class F {
+    public static <T, U> U passNull(Function<? super T, ? extends U> f) {
+        return f.apply(null);
+    }
+}
+
+// FILE: test.kt
+// WITH_RUNTIME
+// FULL_JDK
+fun test(f: (Int?) -> String): String {
+    return F.passNull(f)
+}
+
+fun box(): String {
+    return test {
+        it?.toString() ?: "OK"
+    }
+}