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

The issue points to src/base/ddl_parser.cc line 122. Check the return value and status handling in the DDL parser to ensure that if SQL compilation fails, the function returns null and does not leave partial data in the index_hints map. Review the surrounding code to understand the error flow, then verify the fix by testing with OpenMLDBSQLEmulator.

索引モデルが issue の本文から書いたものです。