Do not overwrite data with null when updating (#275)

[Tolsto]

Problem

Record messages for some updates from certain taps, e.g.
Postgres with log-based replication when doing deletes,
will only contain metadata and pkey columns. The current
MERGE logic would then set all non-included columns in the
target to null.

See #275

Proposed changes

Set the value of columns that were not included in the record message to a random string instead of None. Then, filter out columns with that value in the MERGE statement.

Types of changes

What types of changes does your code introduce to PipelineWise?
Put an x in the boxes that apply

• 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)
• Documentation Update (if none of the other choices apply)

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)
• Branch name starts with AP-NNN (if applicable. AP-NNN = JIRA ID)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions