Update incremental-models.md

[louisdvs]

What are you changing in this pull request and why?

Make it clear that model runs do not fail based on the presence of null values in the unique key.

This distinction is critical as we discovered when a model blew out over time and caused pipeline failure.

Where nulls exist, the merge silently inserts an additional row/s rather than failing the run.

Checklist

• I have reviewed the Content style guide so my content adheres to these guidelines.
• The topic I'm writing about is for specific dbt version(s) and I have versioned it according to the version a whole page and/or version a block of content guidelines.
• I have added checklist item(s) to this list for anything anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

[welcome]

Hello!👋 Thanks for contributing to the dbt product documentation and opening this pull request! ✨
We use Markdown and some HTML to write the dbt product documentation. When writing content, you can use our style guide and content types to understand our writing standards and how we organize information in the dbt product docs.
We'll review your contribution and respond as soon as we can. 😄

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[mirnawong1]

hey @louisdvs , thanks for opening this up and clarifying! we've tweaked it a bit and happy to merge this for you!

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