fix cassandra frontend bug

[LeoQuote]

get_all_columns_by_tb 的返回体之前有问题, 改一下

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d503b55) 75.53% compared to head (d2422b0) 75.53%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2240   +/-   ##
=======================================
  Coverage   75.53%   75.53%           
=======================================
  Files         107      107           
  Lines       15454    15455    +1     
=======================================
+ Hits        11673    11674    +1     
  Misses       3781     3781           

Files Changed	Coverage Δ
archery/settings.py	58.55% <ø> (ø)
sql/engines/cassandra.py	84.56% <100.00%> (+0.10%)	⬆️
sql/engines/test_cassandra.py	90.29% <100.00%> (ø)

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