Rescope empty client suppression

[JoshLove-msft]

In #5218, we implemented logic to skip the generation of clients with no public methods. This has the side effect of preventing customers who customize all client methods from using the helper methods, fields, and properties that would be generated in the client. Since we were implementing this for a very specific known case involving ARM subclients, we should rescope the solution so that we apply it via the ArmVisitor in the Azure Plugin.

[live1206]

Based on the e2e experiment in #5651, the user can implement an empty client in TypeSpec and MGC still generates it.
So, this is not a specific case for Azure plugin only, and then we should skip the empty client generation in MGC as a general behavior.

The criteria of an empty client:

• There is no operation within InputClient
• There is no methods in the corresponding sub-clients
• There is no methods in the corresponding custom code