[PM-2740] Add null check on base64-encoded values on knowndevice query (#3586)

* Added null check on header-based knowndevice call to match query-string implementation.

* Updated to use model binding instead of individual inputs.

* Linting.

src/Api/Models/Request/KnownDeviceRequestModel.cs

@@ -0,0 +1,16 @@
+using System.ComponentModel.DataAnnotations;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Api.Models.Request;
+
+public class KnownDeviceRequestModel
+{
+    [Required]
+    [FromHeader(Name = "X-Request-Email")]
+    public string Email { get; set; }
+
+    [Required]
+    [FromHeader(Name = "X-Device-Identifier")]
+    public string DeviceIdentifier { get; set; }
+
+}