Add missing nil-check on errdefs.Unavailable()

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

errdefs/helpers.go

@@ -76,6 +76,9 @@ func (e errUnavailable) Cause() error {
 
 // Unavailable is a helper to create an error of the class with the same name from any error type
 func Unavailable(err error) error {
+	if err == nil {
+		return nil
+	}
 	return errUnavailable{err}
 }