Use Compount Assignment to respect IDE0054 (#24202)

* Use Compount Assignment to respect IDE0054

Author: Youssef1313

src/Identity/UI/src/Areas/Identity/Pages/V4/Account/Login.cshtml.cs

@@ -109,7 +109,7 @@ public override async Task OnGetAsync(string returnUrl = null)
                 ModelState.AddModelError(string.Empty, ErrorMessage);
             }
 
-            returnUrl = returnUrl ?? Url.Content("~/");
+            returnUrl ??= Url.Content("~/");
 
             // Clear the existing external cookie to ensure a clean login process
             await HttpContext.SignOutAsync(IdentityConstants.ExternalScheme);
@@ -121,7 +121,7 @@ public override async Task OnGetAsync(string returnUrl = null)
 
         public override async Task<IActionResult> OnPostAsync(string returnUrl = null)
         {
-            returnUrl = returnUrl ?? Url.Content("~/");
+            returnUrl ??= Url.Content("~/");
 
             ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();
 

src/Identity/UI/src/Areas/Identity/Pages/V4/Account/Register.cshtml.cs

@@ -126,7 +126,7 @@ public override async Task OnGetAsync(string returnUrl = null)
 
         public override async Task<IActionResult> OnPostAsync(string returnUrl = null)
         {
-            returnUrl = returnUrl ?? Url.Content("~/");
+            returnUrl ??= Url.Content("~/");
             ExternalLogins = (await _signInManager.GetExternalAuthenticationSchemesAsync()).ToList();
             if (ModelState.IsValid)
             {