ddl parser should return null if sql compile failed

https://github.com/4paradigm/OpenMLDB/blob/e1369fb137602621bd41674077c5ab1c46b2d505/src/base/ddl_parser.cc#L122

We should check return value and status. Even the sql compile failed, the map `index_hints` may still have some index hints.

And it effect OpenMLDBSQLEmulator, release a new version, v1.1-beta