Properly handle ErrEmptyClientUnsupportedGet in optimizingClient

[rvagg]

To avoid unwanted logs when you don't need to supply a client and use EmptyClientWithInfo but want drand otherwise setup:

2025-02-04T16:29:52.276+1100    INFO    drand   client/optimizing.go:194                {"optimizing_client": "endpoint down when speed tested", "client": "EmptyClient.(+verifier)", "err": "not supported"}
2025-02-04T16:29:52.276+1100    INFO    drand   client/optimizing.go:194                {"optimizing_client": "endpoint down when speed tested", "client": "EmptyClient.(+verifier)", "err": "not supported"}