Also show empty subtitles

Change-Id: I49d726c57de06db1c6030587737d08ddce4866f6
diff --git a/inst/rmarkdown/templates/revealjs_presentation/resources/default.html b/inst/rmarkdown/templates/revealjs_presentation/resources/default.html
index 853611a..ff05143 100644
--- a/inst/rmarkdown/templates/revealjs_presentation/resources/default.html
+++ b/inst/rmarkdown/templates/revealjs_presentation/resources/default.html
@@ -155,9 +155,7 @@
 $endfor$
   </div>
   <h1 class="title">$title$</h1>
-$if(subtitle)$
   <h2 class="subtitle">$subtitle$</h2>
-$endif$
 $if(date)$
   <div class="place">$date$</div>
 $endif$