Fixed NPE when using logstash appender without any custom fields.

We were getting the following NPE exception when using a logstash syslog appender without any customFields:
RuntimeException in Action for tag [appender] java.lang.NullPointerException
	at java.lang.NullPointerException
	at 	at java.io.StringReader.<init>(StringReader.java:50)
	at 	at net.logstash.logback.encoder.com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:835)
	at 	at net.logstash.logback.LogstashFormatter.parseCustomFields(LogstashFormatter.java:188)
	at 	at net.logstash.logback.LogstashFormatter.setCustomFieldsFromString(LogstashFormatter.java:193)
	at 	at net.logstash.logback.layout.LogstashLayout.setCustomFields(LogstashLayout.java:36)
	at 	at net.logstash.logback.appender.LogstashSocketAppender.buildLayout(LogstashSocketAppender.java:30)
	at 	at ch.qos.logback.core.net.SyslogAppenderBase.start(SyslogAppenderBase.java:64)
	at 	at ch.qos.logback.core.joran.action.AppenderAction.end(AppenderAction.java:96)
	at 	at ch.qos.logback.core.joran.spi.Interpreter.callEndAction(Interpreter.java:317)
	at 	at ch.qos.logback.core.joran.spi.Interpreter.endElement(Interpreter.java:196)
	at 	at ch.qos.logback.core.joran.spi.Interpreter.endElement(Interpreter.java:182)
	at 	at ch.qos.logback.core.joran.spi.EventPlayer.play(EventPlayer.java:62)
	at 	at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:149)
	at 	at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:135)
	at 	at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:99)
	at 	at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java
...

I've added a default JSON array to stop the parser complaining. Please see included test cases.