fix: "Hello None" in the signup email

We were trying to show the full_name, but invite request only knows the email.

community/templates/emails/lms_invite_request_approved.html

@@ -1,6 +1,6 @@
 <div>
 	{% set site_link = "<a href='" + site_url + "'>" + site_name + "</a>" %}
-	<p>{{_("Dear {0},").format(full_name)}}</p>
+	<p>{{_("Hi,")}}</p>
 	<p>{{_("Welcome to {0}!").format(site_name)}}</p>
 	<p>Click on the link below to complete your sign up and set a new password</p>
 	<p style="margin: 15px 0px;">