[TEST] return empty array if AbstractQueryTestCase#currentTypes is null

This is important to allow any test to use RandomQueryBuilder#createQuery()
since some of the query builders that are used in this test test the length
of the types array and otherwise will thow NPE if the test is not a subclass
of AbstractQueryTestCase.

test/framework/src/main/java/org/elasticsearch/test/AbstractQueryTestCase.java

@@ -159,7 +159,7 @@ public abstract class AbstractQueryTestCase<QB extends AbstractQueryBuilder<QB>>
     }
 
     protected static String[] getCurrentTypes() {
-        return currentTypes;
+        return currentTypes == null ? Strings.EMPTY_ARRAY : currentTypes;
     }
 
     protected Collection<Class<? extends Plugin>> getPlugins() {