Fix email settings on store level, when server has no email settings

Fixes btcpayserver#6076.

BTCPayServer.Tests/UnitTest1.cs

@@ -2814,7 +2814,7 @@ await settings.UpdateSetting(new EmailSettings()
                 Password = "[email protected]",
                 Port = 1234,
                 Server = "store.com"
-            }), "", true));
+            }), ""));
 
             Assert.Equal("[email protected]", (await Assert.IsType<StoreEmailSender>(await emailSenderFactory.GetEmailSender(acc.StoreId)).GetEmailSettings()).Login);
         }

BTCPayServer/Controllers/UIStoresController.Email.cs

@@ -195,10 +195,11 @@ public async Task<IActionResult> StoreEmailSettings(string storeId, EmailsViewMo
             if (store == null)
                 return NotFound();
 
-            ViewBag.UseCustomSMTP = useCustomSMTP;
             model.FallbackSettings = await _emailSenderFactory.GetEmailSender(store.Id) is StoreEmailSender { FallbackSender: not null } storeSender
                 ? await storeSender.FallbackSender.GetEmailSettings()
                 : null;
+            if (model.FallbackSettings is null) useCustomSMTP = true;
+            ViewBag.UseCustomSMTP = useCustomSMTP;
             if (useCustomSMTP)
             {
                 model.Settings.Validate("Settings.", ModelState);

BTCPayServer/Views/UIStores/StoreEmailSettings.cshtml

@@ -41,6 +41,7 @@
     }
     else
     {
+        <input type="hidden" name="UseCustomSMTP" value="true" />
         <partial name="EmailsBody" model="Model" />
     }