refactor(generator): remove null check on client description (#4828)

This is a prerequisite change for #3994 The null check for client description is meaningless, because `ClientBuilder` will add a default description anyway: https://github.com/Azure/autorest.csharp/blob/4d55b600ee877e68675297546e70a765fb679039/src/AutoRest.CSharp/Common/Output/Builders/ClientBuilder.cs#L28-L31
Right now in our emitter, we will set `description` to an empty string: https://github.com/Azure/autorest.csharp/blob/4d55b600ee877e68675297546e70a765fb679039/src/TypeSpec.Extension/Emitter.Csharp/src/lib/client-model-builder.ts#L281
But in TCGC, the `description` will be null.

So, let's remove this meaningless check.

src/AutoRest.CSharp/Common/Input/InputTypes/Serialization/TypeSpecInputClientConverter.cs

@@ -49,7 +49,7 @@ public override void Write(Utf8JsonWriter writer, InputClient value, JsonSeriali
             }
 
             name= name ?? throw new JsonException("InputClient must have name");
-            description = description ?? throw new JsonException("InputClient must have description");
+            description = description ?? string.Empty;
             operations = operations ?? Array.Empty<InputOperation>();
             parameters = parameters ?? Array.Empty<InputParameter>();
             var inputClient = new InputClient(name, description, operations, parameters, parent);