internal/godoc/dochtml: only show examples in sidenav if present

This change fixes a bug where the examples section was always shown as a
subsection of Index in the sidenav. Examples is now only shown if that
section exists.

Change-Id: Ic08fca93dedba302a6f53b3cf9dcf01107c8a0fa
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260197
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
diff --git a/internal/godoc/dochtml/template_sidenav.go b/internal/godoc/dochtml/template_sidenav.go
index d5c33da..e02226c 100644
--- a/internal/godoc/dochtml/template_sidenav.go
+++ b/internal/godoc/dochtml/template_sidenav.go
@@ -19,11 +19,13 @@
 							role="treeitem" aria-expanded="false" aria-level="1" aria-owns="nav-group-index" tabindex="-1">
 						Index
 					</a>
-					<ul role="group" id="nav-group-index">
-						<li role="none">
-							<a href="#pkg-examples" role="treeitem" aria-level="2" tabindex="-1">Examples</a>
-						</li>
-					</ul>
+					{{if .Examples.List}}
+						<ul role="group" id="nav-group-index">
+							<li role="none">
+								<a href="#pkg-examples" role="treeitem" aria-level="2" tabindex="-1">Examples</a>
+							</li>
+						</ul>
+					{{end}}
 				</li>
 				<li class="DocNav-constants" role="none">
 					<a href="#pkg-constants" class="js-docNav" role="treeitem" aria-level="1" tabindex="-1">Constants</a>