bug fix. double-star syntax leading to a null pointer exception.

The full stack trace that I had been getting was:
Exception in thread "main" java.lang.NullPointerException
	at org.apache.lucene.util.SimpleStringInterner.intern(SimpleStringInterner.java:54)
	at org.apache.lucene.util.StringHelper.intern(StringHelper.java:39)
	at org.apache.lucene.index.Term.<init>(Term.java:38)
	at org.apache.lucene.queryParser.QueryParser.getWildcardQuery(QueryParser.java:986)
	at org.apache.lucene.queryParser.QueryParser.Term(QueryParser.java:1406)
	at org.apache.lucene.queryParser.QueryParser.Clause(QueryParser.java:1309)
	at org.apache.lucene.queryParser.QueryParser.Query(QueryParser.java:1237)
	at org.apache.lucene.queryParser.QueryParser.TopLevelQuery(QueryParser.java:1226)
	at org.apache.lucene.queryParser.QueryParser.parse(QueryParser.java:206)
	at org.neo4j.index.impl.lucene.IndexType.query(IndexType.java:300)
	at org.neo4j.index.impl.lucene.LuceneIndex.query(LuceneIndex.java:227)
	at org.neo4j.index.impl.lucene.LuceneIndex.query(LuceneIndex.java:238)
	at opentree.GraphExplorer.getSourceList(GraphExplorer.java:1153)
	at opentree.MainRunner.listSources(MainRunner.java:464)
	at opentree.MainRunner.main(MainRunner.java:726)