[gradle] Support nullable system property values

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Description of the PR

The gradle plugin sets all System properties before generation, then reverts them back to their original state.

System.getProperty/setProperty return null if the property was not previously set. The Kotlin map was defined with non-nullable key/value constraints, so setting something not commonly set (modelDocs: "false") would result in an runtime exception.

This changes the map to support nullable values, and rather than setting a null System property at the end, it clears those which previously had no value.

See #551 (closed)