refactor(generator): remove null check on client description

[archerzz]

This is a prerequisite change for #3994 The null check for client description is meaningless, because ClientBuilder will add a default description anyway:

autorest.csharp/src/AutoRest.CSharp/Common/Output/Builders/ClientBuilder.cs

Lines 28 to 31 in 4d55b60

	public static string CreateDescription(string description, string clientPrefix)
	=> string.IsNullOrWhiteSpace(description)
	? $"The {clientPrefix} service client."
	: BuilderHelpers.EscapeXmlDocDescription(description);

Right now in our emitter, we will set description to an empty string:

autorest.csharp/src/TypeSpec.Extension/Emitter.Csharp/src/lib/client-model-builder.ts

Line 281 in 4d55b60

clientDesc = getDoc(program, container) ?? "";

But in TCGC, the description will be null.

So, let's remove this meaningless check.

Description

Add your description here!

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[archerzz]

By the way, it is possible to have a client without description. See

autorest.csharp/test/TestProjects/MixAPIVersion-TypeSpec/MixAPIVersion-TypeSpec.tsp

Line 94 in 4d55b60

namespace ListPetToysResponse {

a namespace can have no @summary