Do not generate @NotNull annotations on void- or primitive-returning methods

#KT-4834 Fixed
  #KT-5255 Fixed

compiler/testData/asJava/lightClasses/nullabilityAnnotations/UnitAsGenericArgument.kt

@@ -0,0 +1,9 @@
+// C
+
+trait Base<T> {
+    fun foo(t: T): T
+}
+
+class C : Base<Unit> {
+    override fun foo(t: Unit) {}
+}