[Bug]: Block clientName to be empty string

[lmazuel]

Describe the bug

Should raise a diagnostic that it's not allowed

Reproduction

import "@azure-tools/typespec-client-generator-core";

using Azure.ClientGenerator.Core;

@clientName("")
model Foo {}

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For bug in the typespec language or core libraries file it in the TypeSpec repo
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• The provided reproduction is a minimal reproducible example of the bug.

[lmazuel]

@timotheeguerin suggested something like:

if(arg === "") {
  reportDiagnostic(program, {
    code: "client-name-blank",
    target: context.getArgumentAtIndex(0)
  })
}

[timotheeguerin]

I would also suggest checking for a whitespace string not just empty string

@clientName(" ") // error on this