Marionette capability should be optional

The marionette capability is a Firefox-specific capability to aid in the
transition from legacy versions of Firefox (47.0.1 and earlier).

It is a `Boolean` and this value is now checked aggressively in Selenium
4.0.0 onwards. Since `null` is not a Boolean, this leads to an error.

This patch removes the default value, which means that if no value is
specified then it will not be included in the output, and the defaults
will apply (True for Firefox; Unset for other browsers).

Without this patch you must jump through hoops to run against Selenium
4.

Fixes Behat#376

src/Behat/MinkExtension/ServiceContainer/Driver/Selenium2Factory.php

@@ -98,7 +98,7 @@ protected function getCapabilitiesNode()
                 ->scalarNode('platform')->end()
                 ->scalarNode('browserVersion')->end()
                 ->scalarNode('browser')->defaultValue('firefox')->end()
-                ->booleanNode('marionette')->defaultNull()->end()
+                ->booleanNode('marionette')->end()
                 ->booleanNode('ignoreZoomSetting')->defaultFalse()->end()
                 ->scalarNode('name')->defaultValue('Behat feature suite')->end()
                 ->scalarNode('deviceOrientation')->end()