Check if thing.childlisting is falsy before including it

This fixes bold `False`s showing up in the `.json-html`
and `.json-compact` renderstyles. Apparently been an issue for
5 years, but it was only noticed now.

r2/r2/templates/comment.compact

@@ -107,5 +107,8 @@
   %else:
     <div class="child">
   %endif
-  ${thing.childlisting}</div>
+  %if thing.childlisting:
+    ${thing.childlisting}
+  %endif
+  </div>
 </div>

r2/r2/templates/comment.html

@@ -129,7 +129,9 @@ ${parent.midcol(cls = cls)}
 %else:
   <div class="child">
 %endif
+  %if thing.childlisting:
     ${thing.childlisting}
+  %endif
   </div>
 </%def>
 

r2/r2/templates/message.compact

@@ -95,7 +95,9 @@
   </div>
   <div style="clear: both;"></div>
   <div class="child">
-    ${thing.childlisting}
+    %if thing.childlisting:
+      ${thing.childlisting}
+    %endif
   </div>
 </div>
 

r2/r2/templates/printable.html

@@ -190,7 +190,9 @@ ${self.RenderPrintable()}
 
 <%def name="Child(display=True)">
 <div class="child" ${(not display and "style='display:none'" or "")}>
-  ${thing.childlisting}
+  %if thing.childlisting:
+    ${thing.childlisting}
+  %endif
 </div>
 </%def>