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

@@ -39,15 +39,16 @@ class AliasCreate(
         else:
             self.domain = self.get_user_domain()
 
-        full_email = f"{self.email}@{self.domain}"
-        if not self.is_email_valid(full_email):
-            error = True
-        elif not self.auth_user.is_admin and self.is_reserved_email(full_email):
-            flash(
-                t("This address is reserved and can only be created by an admin."),
-                category="error",
-            )
-            error = True
+        if not error:
+            full_email = f"{self.email}@{self.domain}"
+            if not self.is_email_valid(full_email):
+                error = True
+            elif not self.auth_user.is_admin and self.is_reserved_email(full_email):
+                flash(
+                    t("This address is reserved and can only be created by an admin."),
+                    category="error",
+                )
+                error = True
 
         enabled = self.request.form.get("enabled")
         if enabled == "on":