Prevent sending empty feedback

technically can still send just a bunch of spaces, but whatever

wuttapos/views/pos.py

@@ -410,6 +410,9 @@ class POSView(WuttaView):
                                    autofocus=True)
 
             def send_feedback(e):
+
+                if message.value:
+
                     self.app.send_email('pos_feedback', data={
                         'user_name': self.page.session.get('user_display'),
                         'message': message.value,
@@ -425,6 +428,10 @@ class POSView(WuttaView):
                     self.page.snack_bar.open = True
 
                     self.main_input.focus()
+
+                else:
+                    message.focus()
+
                 self.page.update()
 
             def cancel(e):