Add Input Validation for Elsa.Identity Login Endpoint (#4601)

* add validator for login request

* login request: only require parameters to be not null (allow empty/whitespace)

src/modules/Elsa.Identity/Endpoints/Login/RequestValidator.cs

@@ -0,0 +1,15 @@
+using FastEndpoints;
+using FluentValidation;
+using JetBrains.Annotations;
+
+namespace Elsa.Identity.Endpoints.Login;
+
+[PublicAPI]
+internal class RequestValidator : Validator<Request>
+{
+    public RequestValidator()
+    {
+        RuleFor(x => x.Username).NotNull();
+        RuleFor(x => x.Password).NotNull();
+    }
+}