Fix false positive "Null can not be a value of a non-null type"

Inferred type of receiver of orElse  is Optional<T & Any>
Generic descriptor is orElse(E!): E!
Substituted descriptor is orElse(T): T , and that is the problem.

Seems that E!  => (T & Any)!  gets expanded to just T  or T & Any , however it should be expanded to
(T & Any) .. (T & Any)?  => T & Any .. T & Any

T & Any is NotNullTypeParameter(T)

The problem is that (T & Any)?  is expanded to T & Any,
that is seems to be wrong.

 #KT-15236 Fixed

compiler/testData/diagnostics/testsWithJava8/nullForOptionalOrElse.kt

@@ -0,0 +1,7 @@
+import java.util.*
+import java.util.stream.Stream
+
+fun <T> Stream<T>?.getIfSingle() =
+        this?.map { Optional.ofNullable(it) }
+        ?.reduce(Optional.empty()) { _, _ -> Optional.empty() }
+        ?.orElse(null) // <<---- should not be an error

compiler/testData/diagnostics/testsWithJava8/nullForOptionalOrElse.txt

@@ -0,0 +1,3 @@
+package
+
+public fun </*0*/ T> java.util.stream.Stream<T>?.getIfSingle(): T?