[client] Answer terminalLoginError's nil case on its own terms

A successful Login reaches terminalLoginError with a nil error, and nothing
covered that. It happens to work on grpc v1.80.0 — gstatus.FromError(nil)
answers (nil, true), and Status.Code tolerates a nil receiver by returning
codes.OK, which is not in the terminal set — but that is a chain of internal
details to be relying on for the common path, and none of it was asserted.

Now the nil error is handled where it is obvious, and the table covers it.

Reported by CodeRabbit on PR #7398, which called it a panic; measured on
v1.80.0 it is not one. The gap was the untested reliance, not a crash.

client/cmd/up_setconfig_refusal_test.go

@@ -74,6 +74,7 @@ func TestTerminalLoginError(t *testing.T) {
 		{name: "daemon unreachable, worth retrying", err: gstatus.Errorf(codes.Unavailable, "connection refused"), wantTerminal: false},
 		{name: "transient internal failure", err: gstatus.Errorf(codes.Internal, "boom"), wantTerminal: false},
 		{name: "not a status error", err: errors.New("boom"), wantTerminal: false},
+		{name: "no error at all, the login succeeded", err: nil, wantTerminal: false},
 	}
 
 	for _, tt := range tests {