Fix HTTP 500 on missing email or domain when creating users and aliases

If the email or domain field was missing or invalid, the error was
flashed, but the address was still built from the unset attributes,
which raised an AttributeError. Only build and validate the address
if both are set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

max/routes/alias/create.py

@@ -39,6 +39,7 @@ class AliasCreate(
         else:
             self.domain = self.get_user_domain()
 
+        if not error:
             full_email = f"{self.email}@{self.domain}"
             if not self.is_email_valid(full_email):
                 error = True

max/routes/user/create.py

@@ -32,6 +32,7 @@ class UserCreate(AllowAdmin, CheckEmailMixin, BackToUsersMixin, BaseTemplateGetV
         else:
             self.domain = domain
 
+        if not error:
             full_email = f"{self.email}@{self.domain}"
             if not self.is_email_valid(full_email):
                 error = True