alps theme: fix nil on emails without a text part

Closes: https://todo.sr.ht/~emersion/alps/100

themes/alps/mailbox.html

@@ -47,7 +47,7 @@
           {{ end }}
           </div>
           <div class="message-list-subject  message-list-item {{ if not (.HasFlag "\\Seen") }}message-list-unread{{ end }}">
-            <a href="{{.TextPart.URL false}}">
+            <a href="{{if .TextPart}}{{.TextPart.URL false}}{{else}}{{.URL}}{{end}}">
               {{if .Envelope.Subject}}
                 {{.Envelope.Subject}}
               {{else}}

themes/alps/message.html

@@ -128,10 +128,10 @@
 
             <span class="followups">
               {{if .Message.HasFlag "\\Draft"}}
-                <a class="action-group button-link" href="{{.Message.URL}}/edit?part={{.Message.TextPart.PathString}}">Edit draft</a>
+                <a class="action-group button-link" href="{{.Message.URL}}/edit{{if .Message.TextPart}}?part={{.Message.TextPart.PathString}}{{end}}">Edit draft</a>
               {{else}}
-                <a class="action-group button-link" href="{{.Message.URL}}/reply?part={{.Message.TextPart.PathString}}">Reply</a>
-                <a class="action-group button-link" href="{{.Message.URL}}/forward?part={{.Message.TextPart.PathString}}">Forward</a>
+                <a class="action-group button-link" href="{{.Message.URL}}/reply{{if .Message.TextPart}}?part={{.Message.TextPart.PathString}}{{end}}">Reply</a>
+                <a class="action-group button-link" href="{{.Message.URL}}/forward{{if .Message.TextPart}}?part={{.Message.TextPart.PathString}}{{end}}">Forward</a>
               {{end}}
             </span>
           </div>