Fix errors arising from short-circuit assumption

The and/or operators in text/template don't short-circuit, so this
seemingly benign change caused a breakage when not fully tested.

themes/alps/message.html

@@ -178,7 +178,9 @@
       {{ $html := .Message.HTMLPart }}
       {{ $text := .Message.TextPart }}
       <div class="tabs">
-        {{ if and $html (ne $html.PathString $text.PathString) }}
+        {{ if and $html $text }}
+        {{ if ne $html.PathString $text.PathString }}
+        {{/* https://github.com/golang/go/issues/31103 */}}
         <a
           href="?part={{.Message.TextPart.PathString}}"
           {{ if eq $text.PathString .Part.PathString }}
@@ -192,6 +194,7 @@
           {{ end }}
         >HTML</a>
         {{ end }}
+        {{ end }}
       </div>
       {{if .View}}
         {{.View}}