fix: bug, allow empty email is registration when it's optional

If the email in the registration form is optional the application should
allow not validate it when it's emtpy. It should only validate the email
when is not empty. We only make sure that if present is a valid email.

src/services/user.rs

@@ -87,12 +87,21 @@ impl RegistrationService {
                         if email.required && registration_form.email.is_none() {
                             return Err(ServiceError::EmailMissing);
                         }
-                        registration_form.email.clone()
+                        match &registration_form.email {
+                            Some(email) => {
+                                if email.trim() == String::new() {
+                                    None
+                                } else {
+                                    Some(email.clone())
+                                }
+                            }
+                            None => None,
+                        }
                     }
                     None => None,
                 };
 
-                if let Some(email) = &registration_form.email {
+                if let Some(email) = &opt_email {
                     if !validate_email_address(email) {
                         return Err(ServiceError::EmailInvalid);
                     }