stop: fix the warn message in case of nil error

looks like it is missed during 0342835
one and when crc is stopped, it show following warn message.
```
WARN Failed to remove crc contexts from kubeconfig: <nil>
Stopped the instance
```

This PR will make sure to handle nil error properly.

Author: praveenkumar

pkg/crc/machine/stop.go

@@ -13,7 +13,7 @@ import (
 func (client *client) Stop() (state.State, error) {
 	defer func(input, output string) {
 		err := cleanKubeconfig(input, output)
-		if !errors.Is(err, os.ErrNotExist) {
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
 			logging.Warnf("Failed to remove crc contexts from kubeconfig: %v", err)
 		}
 	}(getGlobalKubeConfigPath(), getGlobalKubeConfigPath())