Prevent NPE when NullConverter registered

NullConverter tolerates type = null, http://grepcode.com/file/repo1.maven.org/maven2/com.thoughtworks.xstream/xstream/1.4.7/com/thoughtworks/xstream/converters/basic/NullConverter.java#NullConverter
As a result, com.thoughtworks.xstream.core.DefaultConverterLookup.typeToConverterMap get element with key = null and sends it to XvfbEnvironmentConverter.canConvert() when registering XvfbEnvironmentConverter
Merge pull request #5 from aleksz/master

Prevent NPE when NullConverter registered