Fix NPE when parsing source that uses default package

Should fix the following NPE:

Exception in thread "main" java.lang.NullPointerException
	at org.jboss.windup.decorator.java.JavaASTDecorator.processMeta(JavaASTDecorator.java:64)
	at org.jboss.windup.decorator.java.JavaASTDecorator.processMeta(JavaASTDecorator.java:32)
	at org.jboss.windup.decorator.ChainingDecorator.chainDecorators(ChainingDecorator.java:47)
	at org.jboss.windup.interrogator.impl.DecoratorPipeline.processMeta(DecoratorPipeline.java:21)
	at org.jboss.windup.interrogator.impl.ExtensionInterrogator.processMeta(ExtensionInterrogator.java:49)
	at org.jboss.windup.interrogator.impl.ExtensionInterrogator.processFile(ExtensionInterrogator.java:79)
	at org.jboss.windup.interrogator.DirectoryInterrogationEngine.process(DirectoryInterrogationEngine.java:71)
	at org.jboss.windup.WindupMetaEngine.processSourceDirectory(WindupMetaEngine.java:190)
	at org.jboss.windup.WindupMetaEngine.getArchiveMeta(WindupMetaEngine.java:156)
	at org.jboss.windup.WindupReportEngine.generateReport(WindupReportEngine.java:121)
	at org.jboss.windup.WindupMain.processInput(WindupMain.java:144)
	at org.jboss.windup.WindupMain.main(WindupMain.java:71)