Restore old behavior of not setting empty hostnames

The refactoring in flatcar#21
caused hostnames to be set unconditionally compared to the old behavior
of only setting the hostname if it not empty.

When running coreos-cloudinit with datasources that do not provide
metadata such as the `file` datasource, the refactored code caused the
hostname to always be reset to `localhost`. This leads to various
problems like preventing k8s nodes from joining their cluster.

This change restores the old behavior by not applying empty hostnames.

Fixes flatcar/Flatcar#1262

initialize/config.go

@@ -39,6 +39,9 @@ type CloudConfigUnit interface {
 }
 
 func ApplyHostname(hostname string) error {
+	if hostname == "" {
+		return nil
+	}
 	if err := system.SetHostname(hostname); err != nil {
 		return fmt.Errorf("error setting hostname: %w", err)
 	}