fix(fields): apply a fields default value to existing items on creation

[MyvTsv]

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !46151
• Here is a brief description of what this PR does

Root cause

When an admin added a new field with a default value to an existing
container, only items saved afterward received that default value.
Items that already existed:

• kept an empty value in the container table (no retroactive backfill), and
• showed a blank column in search results whenever they had no dedicated
  row at all in the container table, since the search engine only reads
  the raw stored/joined value, with no fallback to the field's default.

Steps to reproduce (before the fix)

1. Create a container on Ticket with a field, and fill it in on an
   existing ticket.
2. Add a new field to the same container with a default value.
3. The existing ticket's new column stays empty instead of showing the
   default.
4. In the Ticket search, add the new field's column: it shows blank for
   tickets with no row at all in the container table, instead of the
   configured default.

Screenshots (if appropriate):