[6.x] Fix grid min_rows breaking CP navigation

[duncanmcclean]

This pull request fixes an issue where a grid field using min_rows could break Control Panel navigation until a hard refresh.

This was happening because preProcess pads empty grids to min_rows using rows with freshly generated _ids, but in two places the meta was built from a different value than the one being returned, so the padded rows had no entry in meta.existing:

• Grid::preload built the new meta without passing along the default row values, so adding a new row containing a nested grid with min_rows produced rows without meta. (Replicator, Bard and Group already pass the defaults along.)
• The field meta endpoint returned the preprocessed value, but built the meta while the field still held the raw value, giving lazily-loaded fields the same mismatch.

Nested fieldtypes then received undefined meta, leaving behind a broken component tree whose unmount throws the Cannot destructure property... error seen in the issue, breaking Inertia navigation until a hard refresh.

This PR fixes it by building the new meta from the default row values, and by setting the preprocessed value on the field before building the meta in the MetaController.

Fixes #14929