Differentiate between empty and nil input

Previously, CopyOf on an empty list was returning nil. With the updates
to SortedUniqueStrings and FirstUniqueStrings, we need to differentiate
between empty lists and nil.

Test: m nothing
Change-Id: I91063ebbe5013cbda5d8f70efde4683c66581599

android/util.go

@@ -26,7 +26,11 @@ import (
 
 // CopyOf returns a new slice that has the same contents as s.
 func CopyOf(s []string) []string {
-	return append([]string(nil), s...)
+	// If the input is nil, return nil and not an empty list
+	if s == nil {
+		return s
+	}
+	return append([]string{}, s...)
 }
 
 // Concat returns a new slice concatenated from the two input slices. It does not change the input