replace empty column name with "column_X"

[A1O]

Fixes #1003

Now all csv files are treated as headed files for preview. This pull request generates a column name "column_X" for empty column name.

Technical details

Other possible solution is to import csv file as headless file by default.

Screenshots

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[kgodey]

We should probably use Column X (no underscore) to be consistent with how we're generating columns on the backend, see #1194

[silentninja]

Can you add a test case to confirm if the PR solves the issue or mention an existing test case that can confirm the behaviour?

[codecov-commenter]

Codecov Report

Merging #1255 (3370370) into master (2e80bd9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1255   +/-   ##
=======================================
  Coverage   93.41%   93.42%           
=======================================
  Files         113      113           
  Lines        4330     4332    +2     
=======================================
+ Hits         4045     4047    +2     
  Misses        285      285           

Flag	Coverage Δ
pytest-backend	93.42% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
db/columns/operations/create.py	91.15% <100.00%> (ø)
db/constants.py	100.00% <100.00%> (ø)
mathesar/imports/csv.py	92.77% <100.00%> (+0.08%)	⬆️

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[A1O]

Can you add a test case to confirm if the PR solves the issue or mention an existing test case that can confirm the behaviour?

@silentninja I added a new unit test.

@kgodey I made the changes you asked for.

[silentninja]

Looks good to me. Nice work! @A1O.