Feature/fix nulls singer decimal

Problem

Should not do a string conversion if the value is None / nulls

Conditional statement to handle this scenario.

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[x] 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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[x] 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
[x] Bumping version in setup.py is an individual PR and not mixed with feature or bugfix PRs
[x] 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