empty unit test csv fixture values default to null

[MichelleArk]

resolves #9881

Problem

CSV fixtures with empty values are converted to "" instead of null -- which is inconsistent with the way seeds work.

Solution

• Convert csv fixture rows from "" to null (None in python, converted to null later down in the framework) if detected.
• Added the normalization to unit test parsing in _validate_and_normalize_rows
• Couldn't find a built-in way to handle this with csv.DictReader (https://stackoverflow.com/a/68305271)

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[MichelleArk]

small formatting issue, unrelated to the behaviour changes in this PR

cc @jtcohen6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.19%. Comparing base (760e4ce) to head (74944d8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10117      +/-   ##
==========================================
+ Coverage   88.16%   88.19%   +0.02%     
==========================================
  Files         181      181              
  Lines       22771    22788      +17     
==========================================
+ Hits        20077    20097      +20     
+ Misses       2694     2691       -3     

Flag	Coverage Δ
integration	85.45% <100.00%> (-0.03%)	⬇️
unit	62.68% <25.00%> (+0.01%)	⬆️

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[github-actions]

The backport to 1.8.latest failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-1.8.latest 1.8.latest
# Navigate to the new working tree
cd .worktrees/backport-1.8.latest
# Create a new branch
git switch --create backport-10117-to-1.8.latest
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 55aad328ea6878ca8bbc5dc53fb734b6142c0ec7
# Push it to GitHub
git push --set-upstream origin backport-10117-to-1.8.latest
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-1.8.latest

Then, create a pull request where the base branch is 1.8.latest and the compare/head branch is backport-10117-to-1.8.latest.