fix: use List<GrantDetails> fallback in Grants page (CS0019)

Agent-Logs-Url: https://github.com/TransactionProcessing/SecurityService/sessions/30e75bb9-e03f-4316-8eb1-6aaf9f516ad3

Co-authored-by: StuartFerguson <16325469+StuartFerguson@users.noreply.github.com>

Author: Copilot

SecurityService/Pages/Account/Grants/Index.cshtml.cs

@@ -33,7 +33,7 @@ public async Task<IActionResult> OnGetAsync(CancellationToken cancellationToken)
         }
 
         var result = await this._mediator.Send(new SecurityServiceQueries.GetUserGrantsQuery(user.Id), cancellationToken);
-        this.Grants = result.Data ?? Array.Empty<GrantDetails>();
+        this.Grants = result.Data ?? new List<GrantDetails>();
         return this.Page();
     }
 
@@ -50,7 +50,7 @@ public async Task<IActionResult> OnPostRevokeAsync(string authorizationId, Cance
         {
             this.StatusMessage = result.Message ?? "The grant could not be revoked.";
             var grantsResult = await this._mediator.Send(new SecurityServiceQueries.GetUserGrantsQuery(user.Id), cancellationToken);
-            this.Grants = grantsResult.Data ?? Array.Empty<GrantDetails>();
+            this.Grants = grantsResult.Data ?? new List<GrantDetails>();
             return this.Page();
         }