doclava: don't print "since api level" string if there's no level available

Change-Id: I20d3a61bcd73bdebcbefc7db3e57f213e04b5802
diff --git a/res/assets/templates/macros.cs b/res/assets/templates/macros.cs
index 2b2490c..f3188f4 100644
--- a/res/assets/templates/macros.cs
+++ b/res/assets/templates/macros.cs
@@ -138,7 +138,7 @@
 
 <?cs # print the API Level ?><?cs
 def:since_tags(obj) ?>
-<?cs if:reference.apilevels ?>
+<?cs if:reference.apilevels && obj.since ?>
   Since: <a href="<?cs var:toroot ?>guide/appendix/api-levels.html#level<?cs var:obj.since ?>">API Level <?cs var:obj.since ?></a>
 <?cs /if ?>
 <?cs /def ?>