[6.x] Fix default values and fieldtype meta after saving entries and terms

[duncanmcclean]

This pull request fixes two issues with the publish form after saving an entry or term and continuing to edit.

Firstly, it fixes an issue where default values weren't applied to fields that had been saved empty (for example, a grid hidden behind a toggle with always_save: false), until the Stache was cleared.

This was happening because Entry::fileData() and Term::fileData() strip null and empty values before writing to disk, but the in-memory object (which is what the save response is built from, and what the Stache caches) keeps them. A hidden grid is processed to [], which isn't null, so Field::preProcess() never fell back to the field's default.

This PR fixes it by mirroring the writer's rule when extracting publish form values: empty values are dropped for root entries and default-locale terms. Localizations are left alone since explicit nulls are meaningful there.

Secondly, it fixes an issue where relationship values set in an EntrySaving or TermSaving listener rendered as broken items after "Save & continue editing", until the page was reloaded.

This was happening because the save pipeline re-applied the response's values, but nothing refreshed the fieldtype meta, so the Relationship fieldtype had no item data for the newly added IDs.

This PR fixes it by returning meta from the entry and term update endpoints and applying it in the save pipeline alongside the values.

The same missing meta was also behind #15418: a conditionally hidden nested Replicator with default: rows gets fresh row IDs on every request, so after saving, the Assets fieldtypes inside those rows couldn't find their meta and threw, leaving the Control Panel unresponsive until a hard refresh.

Fixes #11355
Fixes #11396
Fixes #15418
Fixes #15428
Replaces #11356

[jasonvarga]

The PHP half looks right to me — ExtractsFromEntryFields and ExtractsFromTermFields mirror Entry::fileData()'s isRoot() conditional and Term::fileData()'s default-locale-only stripping precisely. I also confirmed the term restructure doesn't lose in-memory TermSaving changes, since LocalizedTerm::data() and inDefaultLocale()->data() both read through to the same underlying Term.

One issue on the JS side that needs fixing before this goes in — the wholesale setMeta() clobbers client-owned slug meta. Details inline.

Non-blocking: both new PHP tests only cover the root/default-locale branch, so nothing exercises the localization side of the new isRoot() ternary.

Changes were made