add test with flow-sensitive reasoning about nulls

Author: olhotak

tests/explicit-nulls/warn/unnecessary-nn.scala

@@ -10,4 +10,9 @@ def f5a[T <: String](s: T): String = s.nn // warn
 
 // flexible types
 def f7(s: String|Null) = "".concat(s.nn) // warn
-def f8(s: String): String = s.trim().nn // OK because the .nn could be useful as a dynamic null check
+def f8(s: String): String = s.trim().nn // OK because the .nn could be useful as a dynamic null check
+
+
+def f9(s: String|Null): String =
+  if(s == null) "default"
+  else s.nn // warn