Fix rendering unknown values in map and null string primitives

[liamcervante]

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

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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