base HTML: 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

plugins/base/public/mailbox.html

@@ -27,7 +27,7 @@
   <ul>
     {{range .Messages}}
       <li>
-        <a href="{{.TextPart.URL false}}">
+        <a href="{{if .TextPart}}{{.TextPart.URL false}}{{else}}{{.URL}}{{end}}">
           {{if .Envelope.Subject}}
             {{.Envelope.Subject}}
           {{else}}