Backport of Fix rendering unknown values in map and null string primitives into v1.4

[ghost]

Backport

This PR is auto-generated from #33029 to be assessed for backporting due to the inclusion of the label 1.4-backport.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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This PR fixes 2 issues in the new renderer:

1. Unknown values are removed from the set of after values by the json plan package for attributes. This affects maps as they are unaware of the values they are supposed to contain. This PR makes the renderer look at the planned unknown values and merge them back into the main after set before computing the diff.
2. Null strings were being rendered with the %q format which meant they were being encapsulated by quotation marks and the color encodings were being rendered as a string.

This PR fixes both of these issues.

Fixes #33016

Target Release

1.4.6

Draft CHANGELOG entry

BUG FIXES

• Fix bug when rendering plans that include null strings
• Fix bug when rendering plans that include unknown values in maps

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