Fix fallback to NullType when dialect type not found

[Rustem]

GEOGRAPHY custom type is not supported yet. Decided to fix issue similarly how sqlalchemy/postgresql dialect fixes it using sqlalchemy generic type called NULLTYPE. For more information see here

Root cause: sqltypes.NULLTYPE is already an instance of sqltypes.NullType, however when type is not supported logic does not account for that and tries to initalize already initialized instance.

Related Issue: https://github.com/snowflakedb/snowflake-sqlalchemy/issues/194

[Rustem]

Ok tested this workaround on few tables that contain geography and without it, now it works!

[Rustem]

Could you help to merge this bug fix asap please? As we temporarily patched the dialect on our end.

[Mafioso]

We have same problem with dialect. Can you please merge this pull request. @sfc-gh-kwagner @sfc-gh-mkeller

[emreakg]

Any plans to merge this? This looks like a good hotfix to integrate. engine.dialect.get_columns() is unusable due to this if any table in schema contains a GEOGRAPHY column.@sfc-gh-kwagner @sfc-gh-mkeller

[nikhiltjoy-dd]

+1 GEOGRAPHY columns are causing issues

[hadia206]

+1 for merge request. I am getting this error as well when trying to do df.to_sql if any table in schema contains a GEOGRAPHY column.

[njriasan]

+1 for merge request. I am getting this error as well when trying to do df.to_sql if any table in schema contains a GEOGRAPHY column.

+1 on the merge request. I have verified that applying this change resolves my issues.