sourcehut theme: fix nil on emails without a text part

P.a. to 50e131244a which solved
https://todo.sr.ht/~emersion/alps/100

Closes: https://todo.sr.ht/~emersion/alps/102

themes/sourcehut/mailbox.html

@@ -53,7 +53,7 @@
           <li class="nav-item">
             <a
               class="nav-link"
-              href="{{.TextPart.URL false}}"
+              href="{{if .TextPart}}{{.TextPart.URL false}}{{else}}{{.URL}}{{end}}"
             >
             <span class="text-muted date">
               {{ .Envelope.Date | formatdate }}