fix: Issue/644

[karel-rehor]

Closes #644

Proposed Changes

When FluxCSVParser parses a value, and the stringValue is empty and the default value for the column is empty, now checks whether the data type for the column is "string" and if this is the case returns the empty default value instead of null.

Note that headerless csv tables will still return null and that null will be returned if the default value is null.

Two tests are added to FluxCsvParserTest.

Checklist

• [x] CHANGELOG.md updated
• [x] Rebased/mergeable
• [x] A test has been added if appropriate
• [x] mvn test completes successfully
• [x] Commit messages are conventional
• [x] Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (19bde3e) 88.36% compared to head (776c67e) 88.36%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #648 +/- ## ========================================= Coverage 88.36% 88.36% Complexity 779 779 ========================================= Files 172 172 Lines 7022 7024 +2 Branches 382 383 +1 ========================================= + Hits 6205 6207 +2 Misses 698 698 Partials 119 119 ```

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