Default host name override is invalid on my Ubuntu VM (microsoft#172358)

* Default host name override is invalid on my Ubuntu VM

* fix regex

Author: aeschli

src/vs/workbench/contrib/remoteTunnel/electron-sandbox/remoteTunnel.contribution.ts

@@ -752,7 +752,7 @@ Registry.as<IConfigurationRegistry>(ConfigurationExtensions.Configuration).regis
 			description: localize('remoteTunnelAccess.machineName', "The name under which the remote tunnel access is registered. If not set, the host name is used."),
 			type: 'string',
 			scope: ConfigurationScope.MACHINE,
-			pattern: '^\\w[\\w-]*$',
+			pattern: '^(\\w[\\w-]*)?$',
 			patternErrorMessage: localize('remoteTunnelAccess.machineNameRegex', "The name must only consist of letters, numbers, underscore and dash. It must not start with a dash."),
 			maxLength: 20,
 			default: ''