content: add text to versions tab when tagged version is missing

When a series has tagged versions, but the current module version does
not, the versions tab will now display the message "No tagged versions in
this module." instead of displaying nothing.

For golang/go#41345

Change-Id: I2137187532bce18d1c60bc7aa138e14a6fb7e559
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/266141
Trust: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

content/static/css/stylesheet.css

@@ -1368,6 +1368,10 @@ pre {
   font-weight: 400;
   font-size: 1rem;
 }
+.Versions-message {
+  color: var(--gray-3);
+  margin-bottom: 2rem;
+}
 .Versions-modulePath {
   color: var(--gray-3);
   font-size: 1rem;

content/static/html/helpers/_versions.tmpl

@@ -29,7 +29,11 @@
       {{if .OtherModules}}
         <h2>Versions in this module</h2>
       {{end}}
-      {{template "module_list" .ThisModule}}
+      {{if .ThisModule}}
+        {{template "module_list" .ThisModule}}
+      {{else}}
+        <p class="Versions-message">There are no tagged versions of this module.</p>
+      {{end}}
       {{if .OtherModules}}
         <h2>Other modules containing this package</h2>
         {{template "module_list" .OtherModules}}