Fix ArangoDatabase.transaction(): ignore null result

Author: mpv1989

ChangeLog

@@ -1,3 +1,7 @@
+v4.2.x (xxxx-xx-xx)
+---------------------------
+* fixed ArangoDatabase.transaction(): ignore null result
+
 v4.2.3 (2017-07-31)
 ---------------------------
 * added ArangoDatabase.getPermissions(String)

src/main/java/com/arangodb/internal/InternalArangoDatabase.java

@@ -304,7 +304,7 @@ public T deserialize(final Response response) throws VPackException {
 				final VPackSlice body = response.getBody();
 				if (body != null) {
 					final VPackSlice result = body.get(ArangoDBConstants.RESULT);
-					if (!result.isNone()) {
+					if (!result.isNone() && !result.isNull()) {
 						return util().deserialize(result, type);
 					}
 				}