Distinguish between nulls and empty strings

[tombriggsallego]

Problem

Treat empty strings as non-null for varchar fields. Fixes #68

Almost everything I know about Python I learned in order to fix this so there may well be a better way to do it. ;)

Proposed changes

Uses a specific token string to signify NULLs in the COPY command, then doesn't produce that value in the CSV if a given field is a varchar and is an empty string.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
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Checklist

• Description above provides context of the change
• I have added tests that prove my fix is effective or that my feature works
• Unit tests for changes (not needed for documentation changes)
• CI checks pass with my changes
• Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
• Commit message/PR title starts with [AP-NNNN] (if applicable. AP-NNNN = JIRA ID)
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• Relevant documentation is updated including usage instructions