[external-metadata] Fix unknown column types

[john-bodley]

This PR fixes an issue where the SQLAlchemy NullType() cannot be cast to a string. Similar to here the fix is merely to set the type as UNKNOWN.

to: @michellethomas @mistercrunch

[mistercrunch]

LGTM

[codecov-io]

Codecov Report

Merging #6196 into master will decrease coverage by 0.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #6196      +/-   ##
==========================================
- Coverage   76.93%   76.91%   -0.02%     
==========================================
  Files          47       47              
  Lines        9389     9393       +4     
==========================================
+ Hits         7223     7225       +2     
- Misses       2166     2168       +2

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	80.71% <60%> (-0.24%)	⬇️

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